Cleanup variable checks in glance-registry/start.sh

Remove redundant checks for MARIADB_SERVICE_HOST and DB_ROOT_PASSWORD
that are performed in `check_for_db`, and add missing check for
GLANCE_DB_USER.

Change-Id: Ie8194ce76274a9356dcc0b51353873d8a632c66a
Merge "Cleanup variable checks in glance-registry/start.sh"