Cleanup and more tests for db.api methods

New tests for methods:

 backup_*
 quota_*
 reservation_*
 snapshot_*
 transfer_*
 volume_*

Removed unnecessary methods:

 quota_usage_create
 volume_metadata_get_item
 snapshot_metadata_get_item
 volume_type_extra_specs_get_item
 exact_filter

Improved coverage of existing tests.

Improved ModelsObjectComparatorMixin.

blueprint: db-api-tests

Change-Id: Ia2a3845f27a7b4b08fd8a16c6010513d213e642f
Merge "Cleanup and more tests for db.api methods"