restapi: Support labels on networks

Adding support for labels on network level.
Currently the engine supports a single label
per network.

The support actions for labels are GET, POST and DELETE,
done via:

 /api/networks/{network:id}/labels
 /api/networks/{network:id}/labels/{label:id}
 /api/datacenters/{datacenter:id}/networks/{network:id}/labels
 /api/datacenters/{datacenter:id}/networks/{network:id}/labels/{label:id}

 An example of the labels model:
 <network>
   ...
   <labels>
     <label id="lbl1" />
   </labels>
   ...
 <network>

See: http://www.ovirt.org/Features/NetworkLabels#REST

Change-Id: Id8f6f18c6195a2822d87ad626f034cefe43847b4
Bug-Url: https://bugzilla.redhat.com/1040586
Signed-off-by: Moti Asayag <masayag@redhat.com>