Zone Imports - refactoring and new test cases

1) Refactoring:
   - Replace Python underscore variable with indexing.
   - Use constants.
   - Move negative test cases to dedicated "Negative" class.
   - Client's "create_zone_import" had to be improved.
   - Using provided "wait_until" in the existing tests.
2) "test_create_zone_import_invalid_file_data"
   Try to create a zone import using invalid zone file data.
   Expected: zone gets into the ERROR status.
3) "test_zone_cannot_be_update_by_import"
   Try to update an existing zone using Zone import.
   Expected: zone import gets into the ERROR status.
4) "test_create_zone_import_invalid_content_type" to test
   "415 unsupported_content_type" error response.

Change-Id: Ifce7e6ed910c43aacc566cdf692eec45779811e4
3 files changed