Add support for official OVS NSH implementation. Changes include:

1. New nicira fields: nsh_flags, nsh_ttl
2. Different openflow protocol encoding for existing nsh fields, which are now experimenter fields.
3. New nicira actions: encap, decap, dec_nsh_ttl
4. Add support to existing nicira actions to be able to operate on experimenter fields: reg_load, reg_move, output_reg
5. Remove items related to old nsh implementation:
   • pop_nsh, push_nsh actions
   • tun_gpe_np field
   • write support for nsh_mdtype and nsh_np fields.