watchfrr: Allow an integrated config to work within a namespace

watchfrr: Allow an integrated config to work within a namespace

Since watchfrr invokes vtysh to gather the show run output and
write the data, if we are operating inside of a namespace FRR
must also pass this in.

Yes. This seems hacky.  I don't fully understand why vtysh
is invoked this way.

New output:

sharpd@eva:~/frr3$ sudo vtysh -N one

Hello, this is FRRouting (version 8.1-dev).
Copyright 1996-2005 Kunihiro Ishiguro, et al.

eva# wr mem
Note: this version of vtysh never writes vtysh.conf
% Can't open configuration file /etc/frr/one/vtysh.conf due to 'No such file or directory'.
Building Configuration...
Integrated configuration saved to /etc/frr/one/frr.conf
[OK]
eva#

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