bgpd: add show bgp summary filter by neighbor or AS

bgpd: add show bgp summary filter by neighbor or AS

Add ability to filter session on show bgp summary by neighbor or
remote AS:

ubuntu# show bgp summary ?
  neighbor     Show only the specified neighbor session
  remote-as    Show only the specified remote AS session
ubuntu# show bgp summary neighbor ?
  A.B.C.D   Neighbor to display information about
  WORD      Neighbor on BGP configured interface
  X:X::X:X  Neighbor to display information about
ubuntu# show bgp summary remote-as ?
  (1-4294967295)  AS number
  external        External (eBGP) AS sessions
  internal        Internal (iBGP) AS sessions

This patch includes the documentation and the topotest.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>