F-72: Allow update of VM NICs and update disks for consistency

[treywelsh]

This PR allow to update the NICs of a VM in a RUNNING/POWEROFF state.

But overall, the behaviour is the same that for disk update in #59
This PR remove some computed attributes around that were not useful.

I updated helpers that make diffs on list of configs (NIC, disk...)

One change that doesn't impact much the code is that a VM doesn't have transitional states when attach/detach a NIC from a VM in POWEROFF state.
To make less requests we could remove the waitForVMState call in this case.

[treywelsh]

Here is what's happening on this PR: new acceptance tests don't pass due to the fact that NIC schema fields are marked as computed and optional.

This is ok, but this lead to an unexpected behaviour: in case of a NIC update (tf file is modified to detach and attach a single NIC) the diff is broken.
The Get methods return new NIC datas mixed with old values previously computed (on optional fields that weren't set):

2020/11/17 18:21:05 [WARN] Test: Step plan: DIFF:
UPDATE: opennebula_virtual_machine.test
...
  memory:                  "128" => "128"
  name:                    "test-virtual_machine" => "test-virtual_machine"
  nic.#:                   "1" => "1"
  nic.0.ip:                "172.16.100.131" => "172.16.100.111"
  nic.0.mac:               "02:00:ac:10:64:83" => "02:00:ac:10:64:83"
  nic.0.model:             "" => ""
  nic.0.network:           "test-net1" => "test-net1"
  nic.0.network_id:        "0" => "1"
  nic.0.nic_id:            "0" => "0"
  nic.0.physical_device:   "" => ""
  nic.0.security_groups.#: "1" => "1"
  nic.0.security_groups.0: "0" => "0"

Here, the network_id and ip diff is ok, but for the mac (left empty in tf file), the field is not shown as empty and the old mac is kept.

There is a lot of issues around, this one has some clear explanations: hashicorp/terraform-plugin-sdk#282

Some ways I explore to try to fix this:

1. try to handle the diff with customdiff package and RessourceDiff methods:
   I tried SetNewComputed method on diff but doesn't seems to work on nested keys like nic.0.mac...
   Related: SetNew does not work on nested fields hashicorp/terraform-plugin-sdk#459
2. split each attribute in two attributes: one marked as optional, the other as computed
3. introduce opennebula_virtual_machine_nic resource:
   This would be an attachment resource between a VM and a VNet.
   If the ressource require the VM ID, it's not possible to create a VM with attached NIC.
   To solve this, the VM ressource should provide basic NIC management at VM creation (already implemented).
   It's may be possible manage NIC only via the opennebula_virtual_machine_nic resource (even at VM creation) but this lead to some new others problems:
   • an arbitraty ID should be generated for the NIC at opennebula_virtual_machine_nic creation
   • NIC template content should be appended to the VM template at VM creation: this imply data sharing between resources (but I don't know any similar case of serialized data shared among other providers)

This also concern a previous PR already merged about disks #64
For disks I minimized the problem taking into account only the target attribute but it's also marked as optional and computed

[treywelsh]

I see two ways to solve the problem based on 2. and 3. from comment above.

The 2. could be a temporary solution until hashicorp/terraform-plugin-sdk#282 is solved (the problems seems to be that they want to keep backward compatibility with old terraform versions).
Then computed attributes added as a fix could be removed after the migration of the provider to the terraform SDK.

The 3. which introduce a new resource that manage NIC attachement would lead to a new way to describe resource dependencies that solve the problem stated above, but would also partly split NIC (same would apply for disk) management from VM resource.
This would add a bit of complexity (but still much less than in other providers...), but may be a good thing and wouldn't necessarily break the behavior currently implemented in the provider.

Here is the POC branch with new resource and tests: https://github.com/treywelsh/terraform-provider-opennebula/tree/F-72-new-ressource-nic

[treywelsh]

Implement 2.
i.e. split computed+optional attributes in two parts for nic.

There is a bit more to improve overall behavior and for consistency:

• Update generic diff helper (for list of disks or nics) to allow update based on a set of attributes:
  Initially only network_id for nic and image_id were used to trigger attach/detach actions to update the resource.
  Now, for instance, modify disk target will trigger an attach/detach to update the resource.
• disk update code has been updated to work in the same way than NIC