Fix variable error for nova --service-type

When running 'nova' with '--service-type' option, it raises an variable
error.

$ nova --service-type compute list
ERROR: local variable 'os_compute_api_version' referenced before
assignment

When '--service-type' option is given, the default value of
'os_compute_api_version' does not apply.

Change-Id: I19554756e8eb56e3a5984ed3078137af9687f986
Closes-bug: 1315368
Merge "Fix variable error for nova --service-type"