BGP: 'neighbor swpX interface peer-group FOO' is needed to simplify swpX

BGP: 'neighbor swpX interface peer-group FOO' is needed to simplify swpX
configuration

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Dinesh Dutt <ddutt@cumulusnetworks.com>
Ticket: CM-8321

This allows the user to configure the peer-group as an option for the
"neighbor swpX interface" command.
{code}
!
router bgp 100
 neighbor FOO peer-group
 neighbor FOO remote-as external
 neighbor swp1 interface
 neighbor swp2 interface v6only
 neighbor swp3 interface peer-group FOO
 neighbor swp4 interface v6only peer-group FOO
!
 address-family ipv4 unicast
  neighbor FOO activate
  neighbor swp1 activate
  neighbor swp2 activate
 exit-address-family
!
{code}

Note that if the user configures
{code}
neighbor swp5 interface
neighbor swp5 peer-group FOO
{code}

We will display that as "neighbor swp5 interface peer-group FOO".  It
did not seem worth tracking that the peer-group was entered via two
lines instead of one.