Add admin-up flag to L2TP tunnels and start tunnels set to down

Add admin-up flag to L2TP tunnels and start tunnels set to down

L2TP tunnels use virtual interfaces but directly send
packets to l2-input node (not ethernet-input).
This node requires a bridge-domain to be associated with
the interface.

Past code was immediatly turning the interface up, but
some packets could be sent to l2-input without bridge domain
between interface creation and association with a bridge domain.

The tunnel is now created as down and has to be set up later
(typically after being associated with a bridge-domain).

Another option would have been to change the api and enforce
a bridge-domain to be specified before the tunnel, but this
is less flexible for the user.

Change-Id: I26d1f36bb4f327d9fa1c8044023f2210c4117904
Signed-off-by: Pierre Pfister <ppfister@cisco.com>