provider/openstack: Allow empty api_key and endpoint_type

[jtopjian]

These two provider options are optional though if they are not set,
the user will be prompted to enter values.

By changing them to use the envDefaultFuncAllowMissing, the values
are still passed in the environment if they are set and safely
discarded if they are not.

[jtopjian]

This is definitely a bug fix rather than an enhancement (should I have opened a corresponding issue?). OpenStack deployments using Keystone v2 will receive an error when OS_AUTH_TOKEN is set:

$ terraform plan
Refreshing Terraform state prior to plan...

Error refreshing state: 1 error(s) occurred:

* The base Identity V2 API does not accept authentication by APIKey

The way it's currently configured, the user will be prompted to enter an API key and thus be caught in a catch-22.

OS_ENDPOINT_TYPE, while not causing breakage when set, is very much an optional setting. As seen in e04a6ca, it easily falls back to a default, working value, so being prompted to enter it is more of a hassle.

@phinze @mitchellh: May I ask a favor and get this reviewed in time for the next release?

[ggiamarchi]

LGTM

[phinze]

LGTM - thanks @jtopjian!

[ghost]

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