Improve openstacklib::db::mysql test coverage

* Add tests for openstacklib::db::mysql::host_access
* Add tests for other overridable parameters in db::mysql
* Do not test implementation details of host_access from
  within openstacklib::db::mysql tests.

Change-Id: Ifcf3820a575f932313a62b9d294ebd92a5055cf5
Merge "Improve openstacklib::db::mysql test coverage"