bgpd: Added hidden CLI command to disable sending of End-of-Rib.

bgpd: Added hidden CLI command to disable sending of End-of-Rib.

BGP disable EOR sending is a useful command for testing various
scenarios of BGP graceful restart.
* Added the hidden CLI command :  bgp graceful-restart disable-eor
* The CLI will not be displayed in "show running-config" and will not
  be stored in configuration file.
* When enabled, EOR will not be sent to peer

Signed-off-by: Biswajit Sadhu <sadhub@vmware.com>
Signed-off-by: Soman K S <somanks@vmware.com>