openstack get_instances version check moved into stats matching function

We were checking the appliance version in openstack's method for getting a list of instances, which we do when we're trying to deploy an openstack image, which interferes with configuring openstack appliances.

This is related to PR #1313, in that this is the next thing I ran into after merging the fix there.

After making this change, stats matching works on 5.2 and 5.3, but the openstack mgmt_sys will always list all VMs. I'm interested to see what the PR tester says about this one. :)