Cleanup copy/paste in test_quota_sets

Tests in test_quota_sets extensively use quota_set data that's
copy/pasted everywhere, so introduce a method that generates this
quota_set data and make use of it in the tests.

blueprint nova-tests-code-duplication

Change-Id: Icac3e4b5f48592c63a8376c84a744f3dac0cec36
Merge "Cleanup copy/paste in test_quota_sets"