As shown in documentation: this is optional, but if it is set, it
does the following things.
1) Allows DHCP hosts to have fully qualified domain names, as long
as the domain part matches this setting.
2) Sets the "domain" DHCP option thereby potentially setting the
domain of all systems configured by DHCP
3) Provides the domain part for "expand-hosts"
Under certain cases, have FQDN in servers is a need.
Change-Id: Id8a109984c13e95841d9e0f283d55261d083542c
2d713268a3