session: Add sock_name option to add_ns

session: Add sock_name option to add_ns

This adds a new API call to add session namespaces
It now takes a netns and a sock_name.
(1) If no netns is passed, sock_name will be used as
socket path. Defaulting to /run/vpp/app_ns_sockets/${ns_id}
(2) If a netns is passed, the sock_name has to be
abstract (i.e. start with '@'). It will default to
`@vpp/session/${ns_id}` and will be created in the provided
netns.

Type: feature

Change-Id: I90e9a8e5ecca2cabe7c05335663e33c8506dc9e7
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>