gnocchi::db: Do not test behavior of lower libraries
gnocchi::db: Do not test behavior of lower libraries

Current unit tests of gnocchi::db tests behavior of puppetlabs-mysql
and puppetlabs-postgresql but this is not necessary and makes our
maintenance difficult because we need to update our tests according
to changes in lower libraries.

This makes these unit tests assert only interfaces and stop testing
behavior of these classes imported from libraries.

Change-Id: I94744b06d37a3a6b28bf8ea09c1096e454676b12

openstack/puppet-gnocchi,openstack/puppet-gnocchi