Verify ASN in OriginVn extended community

Remove a receieved OriginVn extended community if the AS number
is not the same as ours. Derive the OriginVn from route targets
instead.

This change is required for correct operation in a multiple VNC
scenario.

RemoveOriginVnAndLocate now takes an ExtCommunityValue instead
of an ExtCommunityList.

Change-Id: I030bc0feb8ed7ec5e4d24caa82c66a16e5642626