Since:
- novaclient doesn't require specify the *compute api* version
(default is 2.latest now)
- novaclient doesn't use COMPUTE_API_VERSION, since it's wrong name(
OS_COMPUTE_API_VERSION is a correct name)
we can remove COMPUTE_API_VERSION and NOVA_VERSION vars
Change-Id: I47856863e9403870b8d60c778b97d3de1a212ae1