postgresql_user: make password optional for new users

This allows the creation of group roles not meant for logging in at all, as
well as the creation of user roles meant to be authenticated via different
means (LDAP, ...)

Removing an existing password is currently not possible by means of the
"password" argument. (One could specify "PASSWORD NULL" via the
"role_attr_flags" param, but this might subvert the intended use of
role_attr_flags, therefore I don't think that's a good idea)