Return True if the DB user already exists.

def db_user_exists(username):
    """Return True if the DB user already exists.
    """
    qry = u"""SELECT COUNT(*) FROM pg_roles where rolname = \'{username}\';"""
    output = StringIO()
    excute_query(
        qry.format(username=username),
        flags="-Aqt",
        use_sudo=True,
        stdout=output
    )
    # FIXME: is there a way to get fabric to not clutter the output
    # with "[127.0.0.1] out:" on each line?
    lines = output.getvalue().splitlines()
    return lines and lines[0].endswith('out: 1')