VMM: renamed function chk_domainname() -> check_domainname().
Moved EmailAddress.check_localpart() -> VirtualMailManager.check_localpart().
Some small code cleanups in class EmailAddress.
VMM/Domain: added function get_gid() to the Domain module.
We don't need to load all the domain related information from the
database, when we need only the GID of a domain. For example in the
Alias or Relocated classes.