Fix inherits issue with validate_db_connection

Previously the validate_db_connection defined type was trying to use inherits
like a class. This of course would fail.

After analyzing its usage, I've removed the need for the top-level params
inherit by just including the 'postgresql' module which pulls in the client
package and the params class as well. It also avoids resource duplication
for the client package as well.

To ensure we don't get regression on this I've added system tests that test
this defined type in a success and failure state.

Signed-off-by: Ken Barber <ken@bob.sh>