pimd: setup multicast vxlan tunnel termination device

pimd: setup multicast vxlan tunnel termination device

An interface needs to be designated as "termination device" and added to
the termination mroute's OIL. This is used by kernel and ASIC backends
to vxlan-decaps matching flows.

The default termination device is expected to have the prefix (start
sub-string) "ipmr-lo". This can be made configurable if needed -
root@TORS1:~# ip -d link show ipmr-lo
28: ipmr-lo: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN mode DEFAULT group default
    link/ether 12:5a:ae:74:51:a2 brd ff:ff:ff:ff:ff:ff promiscuity 0
    dummy addrgenmode eui64
root@TORS1:~# ip mr

This commit includes the changes to enable pim implicitly on the device
and set it up as the vxlan-term device per-pim-instance.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>