Check minimum version for CLI tests

This is my use case:

We package OpenStack internally and right now we're packaging stable/icehouse server packages (i.e. nova) plus the clients based on required versions around early April before the Icehouse release, so we package python-novaclient-2.17.0 right now.

With branchless Tempest, a CLI test was added for the nova server-group-list API:

https://github.com/openstack/tempest/commit/75ecdfb2083321dacfd4182c9108172452e4f010

However, that command didn't get into python-novaclient in the 2.17 release, it's on trunk:

https://github.com/openstack/python-novaclient/commit/c40891b2824805458db40067ab669d961ecfdfed

So we fail when trying to run that test with python-novaclient-2.17 since the command isn't available there.

Tempest doesn't hit a problem in the upstream CI since it's testing against trunk code for the clients rather than released versions on pypi.

My proposal here is that we add a @minversion decorator for the Tempest CLI tests such that a minimum version of the client is required for the test to run, otherwise it's skipped.

This would still test the new CLIs against unreleased versions of the clients since the in-development version of the packages should be higher than the minimum based on the git hash and how pbr doesn't packaging, but shouldn't fail for packagers/deployers that are only testing stable releases of the server packages and don't have CI on unreleased versions of the clients.