Add resource provider allocations API for placement

[abnvanand]

For #526

Links to the line numbers/files in the OpenStack source code that support the
code in this PR:

API doc:
https://docs.openstack.org/api-ref/placement/?expanded=list-resource-provider-usages-detail#list-resource-provider-allocations

API code:
https://github.com/openstack/placement/blob/2150dbf6880c379b52035b604c51048743c446a5/placement/handlers/allocation.py#L271

[abnvanand]

Hi @jtopjian. This PR adds API to get allocations for a resource provider.
Can you please review?

Thanks

[theopenlab-ci]

Build succeeded.

• gophercloud-unittest : SUCCESS in 3m 46s
• gophercloud-acceptance-test : SUCCESS in 2h 26m 30s
• gophercloud-acceptance-test-ironic : SUCCESS in 22m 59s

[jtopjian]

I think this is the best representation of the schema: https://github.com/openstack/placement/blob/6f00ba5f685183539d0ebf62a4741f2f6930e051/placement/schemas/allocation.py#L75-L126

It also details a project_id and user_id, however, from looking at the acceptance test output, those are not used in the final response.

[jtopjian]

@abnvanand Looks like this needs gofmt run on it. Other than that, this looks good to me and I can merge once that's fixed.

[abnvanand]

Fixed formatting.

[theopenlab-ci]

Build succeeded.

• gophercloud-unittest : SUCCESS in 3m 47s
• gophercloud-acceptance-test : SUCCESS in 1h 11m 22s
• gophercloud-acceptance-test-ironic : SUCCESS in 23m 04s

[jtopjian]

LGTM - Thank you!

[jtopjian]

@abnvanand Thanks so much for working on this.

[coveralls]

Coverage increased (+0.01%) to 79.877% when pulling 306d9e0 on abnvanand:placement-allocations into 2841b2e on gophercloud:master.