Replace assertEqual(None, *) with assertIsNone in tests

Replace assertEqual(None, *) with assertIsNone in tests to have
more clear messages in case of failure.

Change-Id: I6deee90c31adf61d80e2678a5f29ba9e187281c9
Remove None for dict.get()

Because If no default value is specified it defaults to None already.

Change-Id: I3caad9f17840347e30465c7bd4c9c4fe53d991e3
Invalid client version message unclear

The error message when using an invalid os-compute-api-version is
created by joining all the available versions without space.
This patch will add ', ' between versions.

Ex: nova --os-compute-api-version v3 list
ERROR: Invalid client version 'v3'. must be one of: 321.1
Now: nova --os-compute-api-version v3 list
ERROR: Invalid client version 'v3'. must be one of: 3, 2, 1.1

Change-Id: I825df89d38adc9e4bd3fb900cd0199f159c04a6b
Merge "Invalid client version message unclear"
Merge "Replace assertEqual(None, *) with assertIsNone in tests"