nsh: rework NSH netlink keys and actions

nsh: rework NSH netlink keys and actions

This patch changes OVS_KEY_ATTR_NSH
to nested attribute and adds three new NSH sub attribute keys:

    OVS_NSH_KEY_ATTR_BASE: for length-fixed NSH base header
    OVS_NSH_KEY_ATTR_MD1:  for length-fixed MD type 1 context
    OVS_NSH_KEY_ATTR_MD2:  for length-variable MD type 2 metadata

Its intention is to align to NSH kernel implementation.

NSH match fields, set and PUSH_NSH action all use the below
nested attribute format:

OVS_KEY_ATTR_NSH begin
    OVS_NSH_KEY_ATTR_BASE
    OVS_NSH_KEY_ATTR_MD1
OVS_KEY_ATTR_NSH end

or

OVS_KEY_ATTR_NSH begin
    OVS_NSH_KEY_ATTR_BASE
    OVS_NSH_KEY_ATTR_MD2
OVS_KEY_ATTR_NSH end

In addition, NSH encap and decap actions are renamed as push_nsh
and pop_nsh to meet action naming convention.

Signed-off-by: Yi Yang <yi.y.yang@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>