Cleanup _interface class variables in compute

We've already removed these variables in Nova v3 tests.
  Change-Id: Ie9ad2242d20ce90fca74ca7a5390e0971f830caa
But we can remove these not only v3 tests but also v2 tests. This commit
sets _interface = 'json' to BaseV2ComputeTest and removes redundancy.

Change-Id: I10110b92ed9a34eedef210ad8408f78d64ba4ee6
Merge "Cleanup _interface class variables in compute"