bgpd: Add command to prefer global ipv6 address

bgpd: Add command to prefer global ipv6 address

There are cases where customers desire the ability to override the
default behavior of installing ipv6 prefixes with a link-local next-hop
if both a link-local and global ipv6 next-op is present in the bgp table.
This fix provides this ability and will allow the global to be used as the
next-hop.  This also retains the ability to manually set the ipv6 next-hop
global value as before, and if so, this manual entry will be used for the
next-hop.

Ticket: CM-11480
Signed-off-by: Don Slice
Reviewed By: CCR-4983
Testing Done: Manual testing results attached to the ticket. bgp-min and
bgp-smoke will be completed before committing.