Add missing tests for cell_* methods

There was no tests in test_db_api for cell_*
methods.
Also small fix CellNotFound exception.
Add tests to ensure that all works.

blueprint db-api-tests

Change-Id: I63f3a449238f870fe2549ea5c008b636104e5002
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp