Cleanup _interface class variables in compute/v3

We've already removed xml interface from Nova V3 APIs. So we can assume
the default is json. This commit sets _interface = 'json' to
BaseV3ComputeTest class and removes redundancy.

Change-Id: Ie9ad2242d20ce90fca74ca7a5390e0971f830caa
Remove unused _parse_array functions

They are not used in the current code. So they should be removed.

Change-Id: I6d72d12b1ed55fcf7609aaaa4c3d49b6b2b75842
Merge "Remove unused _parse_array functions"
Merge "Cleanup _interface class variables in compute/v3"