core: [RFE] Display Address Override

This patch introduces the possibility to override
the display address of the host by an IP or FQDN

Detailed description:
http://wiki.ovirt.org/wiki/Features/Display_Address_Override

- vds_static enriched by a new field console_address
- if it is null, it means the address has not been overridden
- if it is not null, the address has been overridden and all
  VMs on this host will be considered to have the display
  address to be the the one in defined in console_address
- the vds_static can be a valid FQDN or IP or null - to
  support this a new validator NullEnabledHostnameOrIp has
  been introduced

Change-Id: Idacc0161a104c1de0969d86d98ddd5458c6787b8
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>