testing for existence of galaxy db fails for restrictive postgres setups

[bernt-matthias]

sqlalchemy-util tries to connect to a db called 'postgres' for testing the existence of the galaxy data base (it also does for creation and dropping the galaxy db). If there is no connect privilege for this DB we get:

Queue Worker on sqlalchemy+postgresql://********:********@postgres.intranet.ufz.de/galaxy-dev
Traceback (most recent call last):
  File "lib/galaxy/webapps/galaxy/buildapp.py", line 49, in app_factory
    app = galaxy.app.UniverseApplication(global_conf=global_conf, **kwargs)
  File "lib/galaxy/app.py", line 89, in __init__
    self._configure_models(check_migrate_databases=self.config.check_migrate_databases, check_migrate_tools=check_migrate_tools, config_file=config_file)
  File "lib/galaxy/config.py", line 1165, in _configure_models
    create_or_verify_database(db_url, config_file, self.config.database_engine_options, app=self)
  File "lib/galaxy/model/migrate/check.py", line 33, in create_or_verify_database
    new_database = not database_exists(url)
  File "/gpfs1/data/galaxy_server/galaxy-dev/.venv/lib/python2.7/site-packages/sqlalchemy_utils/functions/database.py", line 481, in database_exists
    return bool(get_scalar_result(engine, text))
....
  File "/gpfs1/data/galaxy_server/galaxy-dev/.venv/lib/python2.7/site-packages/psycopg2/__init__.py", line 130, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
OperationalError: (psycopg2.OperationalError) FATAL:  permission denied for database "postgres"
DETAIL:  User does not have CONNECT privilege.

For usegalaxy-eu the solution is this: usegalaxy-eu@59305ef

Maybe the following PR for sqlalchemy-utils brings a better solution kvesteri/sqlalchemy-utils#372