Identity V3: missing api coverage for 'Get Endpoint' method

add missing test for the following method
https://specs.openstack.org/openstack/keystone-specs/api/v3/identity-api-v3.html#get-endpoint

Change-Id: I4f5a10b3cedf423d1bd908886ffcf44a5c23928c
Closes-Bug: #1536725
2 files changed