Add OpenStack metadata grains module

[zmingee]

What does this PR do?

This PR adds a grains module for the OpenStack metadata service. Metadata from Nova (meta_data.json) and Neutron (network_data.json) will be exposed on the minions grains under the openstack key.

Note that while OpenStack does support Amazon EC2-compatible metadata (which can be exposed using the extant metadata grains module), not all data is provided there, and some OpenStack-specific data is reserved for it's own API.

Previous Behavior

OpenStack metadata was not exposed via grains.

New Behavior

OpenStack metadata is now exposed via grains.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [Y] Docs
• [Y] Changelog - https://docs.saltstack.com/en/master/topics/development/changelog.html
• [Y] Tests written/updated

Commits signed with GPG?

Yes

Please review Salt's Contributing Guide for best practices.

See GitHub's page on GPG signing for more information about signing commits with GPG.

[whytewolf]

@zmingee can we get a basic docstring on get_metadata that is what the main failure in the checks is about.

also you might want to run pre-commit on your local copy. just to make sure that it clears so that check passes.

[MKLeb]

@zmingee have you been able to circle back to this one?

[dwoz]

Closing this due to inactivity. Anyone should feel free to re-open it if they want to see it through to the end in one release cycle.