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.
.. Substitutions in section 5..|account.password_length|replace::**password_length**..|misc.password_scheme|replace::**misc.password_scheme**..|vmm configure|replace::**vmm configure**.._`vmm configure`: vmm.1#configure..|domainadd|replace::**domainadd**.._domainadd: vmm.1#domainadd..|domaindelete|replace::**domaindelete**.._domaindelete: vmm.1#domaindelete..|useradd|replace::**useradd**.._useradd: vmm.1#useradd..|userdelete|replace::**userdelete**.._userdelete: vmm.1#userdelete..|userinfo|replace::**userinfo**.._userinfo: vmm.1#userinfo.. non vmm..|virtual_alias_expansion_limit|replace::`virtual_alias_expansion_limit`.._virtual_alias_expansion_limit: http://www.postfix.org/postconf.5.html#virtual_alias_expansion_limit.. #EOF