Add a check for compute api versions to verify_tempest_config

This commit adds another verification method to the
verify_tempest_config script that adds a check to see if the compute
v3 api is enabled.

Partially implements bp config-verification

Change-Id: Idae7db73c9eba94b7f74ec2f71e20d4267aa582c
Merge "Add a check for compute api versions to verify_tempest_config"