zebra: Properly unregister hook on shutdown

zebra: Properly unregister hook on shutdown

the zebra pseudo wire code was registering a callback
per vrf.  These callbacks are not per vrf based.  They
are vrf agnostic so this was a mistake.  Modify the code
to on startup register once and on shutdown unregister once.

Finally rename the zebra_pw_init and zebra_pw_exit functions
to more properly reflect when they are called.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>