Admin dbi

CREATE DATABASE and DROP DATABASE are now done with a DBI connection instead of running the postgres tools from the command line.

This branch also removes some obsolete cruft.
is_valid_role() moves from TestDbServer/Command/CreateDatabaseFromTemplate.pm to PostgresInstance in commit 7047a21.  By the end of this branch, all grant_role_to_superuser() does is check that the roles involved are valid with is_valid_tole() and then calls grant_role_to_role(); no SQL.