Improve testing of list_extensions for compute

This commit takes the testing for list_extension from the volumes
extensions client and uses it for the compute testing of
list_extensions. This also fixes a skip decorator to use use
requires_ext instead of calling is_extension_enabled directly.

Change-Id: Id08f39e09df0aea0d3c8f507f421c82ddf32900f
Merge "Improve testing of list_extensions for compute"