vat2: add shared memory argument

vat2: add shared memory argument

Add prefix argument to specifiy shared memory segment.
Add long arguments.
Add argument to dump apis.
Add help.
Add template argument

E.g:
vat2 --template sw_interface_add_del_address
{
	"_msgname":	"sw_interface_add_del_address",
	"sw_if_index":	0,
	"is_add":	false,
	"del_all":	false,
	"prefix":	"0.0.0.0"
}

Usage: vat2 [OPTION] <message-name> <JSON object>
Send API message to VPP and print reply

-d, --debug       Print additional information
-p, --prefix      Specify shared memory prefix to connect to a given VPP instance
-f, --file        File containing a JSON object with the arguments for the message to send
--dump-apis       List all APIs available from connected VPP instance

Type: improvement
Signed-off-by: Ole Troan <ot@cisco.com>
Change-Id: I2d32483a727bc16990c9a30dfa9bc1fa7b1fa85a
Signed-off-by: Ole Troan <ot@cisco.com>