zebra: add 'show router-id'

zebra: add 'show router-id'

router-id is buried deep in "show running-config", this new
command makes it easy to retrieve the user configured router-id.
Example:

  # configure terminal
  (config)# router-id 1.2.3.4
  (config)# end

  # show router-id
  router-id 1.2.3.4

  # configure terminal
  (config)# no router-id 1.2.3.4
  (config)# end

  # show router-id
  #

Signed-off-by: Jafar Al-Gharaibeh <jafar@atcorp.com>