Add service/endpoint discover to verify_tempest_config

This commit adds a check to verify_tempest_config for which services
are available. This is used to do 2 things:

1. Verify that the entries in the service_availble group are set correctly
2. Use for filtering which checks inside the script are valid to perform

Partially implements bp config-verification

Change-Id: Ib0e55f21b83d0746994e7e7e1f9666524e1bb3b7
Merge "Add service/endpoint discover to verify_tempest_config"