Support MPLSoGRE with the new FIB 2.0

Support MPLSoGRE with the new FIB 2.0

Note that the new way to provision MPLSoGRE is using the "normal"
GRE tunnel such as in the following example:
  create gre tunnel src 10.0.3.1 dst 10.0.3.3
  set in state gre0 up
  set int ip addr gre0 10.0.4.1/30
  set int mpls gre0 enable
  ip route table 4 6.0.0.0/24 via 10.0.4.2 gre0 out-label 30
  mpls local-label add 30 eos ip4-lookup-in-table 4

The previous CLIs/APIs used to configure MPLSoGRE tunnel such as
"create mpls gre tunnel...", "mpls encap|decap add label .." and
"show mpls tunnel", etc. can not be used. They will be deprecated
in a later update to the VPP code base.

Change-Id: I244916841924dc2b87d2143691cd8476716c06b1
Signed-off-by: John Lo <loj@cisco.com>