Hard-code stubbed novaclient api_version to 2.1
Hard-code stubbed novaclient api_version to 2.1

The unit tests fail with novaclient 6.0.0 which requires
the use of an api_version property on the base Client class.

This change hard-codes the stubbed api_version attribute to
be 2.1 which is equivalent to the preferred version used in
the actual openstack_dashboard.api.nova.VERSSIONS code.

Change-Id: I9b89a224ad715cd0933d9757ca70e8a577eefc95
Closes-Bug: #1619093

BiznetGIO/horizon,ChameleonCloud/horizon,noironetworks/horizon,ChameleonCloud/horizon,sandvine/horizon,yeming233/horizon,ChameleonCloud/horizon,sandvine/horizon,bac/horizon,BiznetGIO/horizon,openstack/horizon,yeming233/horizon,noironetworks/horizon,yeming233/horizon,sandvine/horizon,yeming233/horizon,coreycb/horizon,openstack/horizon,bac/horizon,NeCTAR-RC/horizon,BiznetGIO/horizon,coreycb/horizon,noironetworks/horizon,NeCTAR-RC/horizon,openstack/horizon,NeCTAR-RC/horizon,BiznetGIO/horizon,coreycb/horizon,ChameleonCloud/horizon,coreycb/horizon,openstack/horizon,sandvine/horizon,NeCTAR-RC/horizon,bac/horizon,noironetworks/horizon,bac/horizon