zebra: add 3 new gre commands, and enforce synchro mecanism

zebra: add 3 new gre commands, and enforce synchro mecanism

3 new gre commands are available:
- GRE_GET to permit a daemon to retrieve gre information.
- GRE_UPDATe is the reply message from zebra to the daemon. as it is a
  syncronous request, the GRE_GET expected will have to match the vrf id
  where the gre information is wished. this has an impact on label
  manager with change in APIs.
- SET_GRE_SOURCE. this command will be stubbed for now, assuming that
  the gre interface is set accordingly by external script.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>