provider/vsphere: Change ip_address parameter for ipv6 support

[tkak]

This is a proposal PR to fix #3957.

I'd like to split ip_address parameter to ipv4_address and ipv6_address to support IPv6 address. (But, current version can not configure IPv6 address, just only read IPv6 address.)
And I'm thinking about changing subnet_mask parameter to ipv4_prefix_length and ipv6_prefix_length to simplify a configuration file and the code.

Before:

resource "vsphere_virtual_machine" "foo" {
    name = "sample"
    datacenter = "foo-dc"
    cluster = "foo-cluster"
    vcpu = 2
    memory = 4096
    gateway = "10.71.128.254"
    network_interface {
        label = "foo-label"
        ip_address = "10.71.128.1"
        subnet_mask = "255.255.255.0"
    }
    disk {
        datastore = "foo-datastore"
        template = "foo-template"
    }
}

After:

resource "vsphere_virtual_machine" "foo" {
    name = "sample"
    datacenter = "foo-dc"
    cluster = "foo-cluster"
    vcpu = 2
    memory = 4096
    gateway = "10.71.128.254"
    network_interface {
        label = "foo-label"
        ipv4_address = "10.71.128.1"
        ipv4_prefix_length = 24
    }
    disk {
        datastore = "foo-datastore"
        template = "foo-template"
    }
}

Thanks!

[chrislovecnm]

@tkai I like the design and the changes. LGTM

@phinze thoughts?

[phinze]

@tkak similar to how we did with the vcenter parameter, can we keep the old parameters as deprecated for a release so we don't break existing users config without warning them?

[tkak]

@phinze OK, I'll try to fix.

[tkak]

@phinze I tried to fix for the old parameters. Could you check that?

[phinze]

This shouldn't be accessing the internal storage format of network_interface, I'll follow up post-merge with a commit tweaking this and cc you so you can see what I mean. 👍

[phinze]

Looks good with one tweak I'll make on master. Thanks @tkak!

[ghost]

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