Add missing tests for certificate_* methods

There was no tests in test_db_api for certificate_*
methods.

Add tests to ensure that all works.

blueprint db-api-tests

Change-Id: I1ef8908d723676e6109a5711e2f8cb439e5f2985
Merge "Add missing tests for certificate_* methods"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp