pim-anycast-rp: Support in BGP unnumbered networks.

pim-anycast-rp: Support in BGP unnumbered networks.

Anycast rp requires multiple ip addresses on the lo. If PIM is used in
an unnumbered BGP network it picks one of the lo addresses as the
pim-primary for the swp interfaces. But if the anycast IP is picked up
by both sides pim nbr will never converge. So a static "use-source" config
is provided to allow the administrator to force the the hello source to the
unique IP address.

Sample output:
=============
dell-s6000-04(config-if)# do show running-config pimd
>>>>>> SNIPPED >>>>>>>>>>>>>>>>>
interface lo
 ip pim sm
 ip pim use-source 100.1.1.5
!
>>>>>> SNIPPED >>>>>>>>>>>>>>>>>
dell-s6000-04(config-if)# do show ip pim interface lo
Interface  : lo
State      : up
Use Source : 100.1.1.5
Address    : 100.1.1.5 (primary)
             100.1.1.100
>>>>>> SNIPPED >>>>>>>>>>>>>>>>>
dell-s6000-04(config-if)# do show ip pim interface lo json
{
  "lo":{
    "name":"lo",
    "state":"up",
    "address":"100.1.1.5",
    "index":1,
    "lanDelayEnabled":true,
    "useSource":"100.1.1.5",
    "secondaryAddressList":[
      "100.1.1.100"
    ],
>>>>>> SNIPPED >>>>>>>>>>>>>>>>>

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>