lib: zebra support for vrfwnetns parameter

lib: zebra support for vrfwnetns parameter

topotest library is being added a new parameter when preparing the
configurationof each daemon. This parameter will be used to call the
daemon with some extra parameters. The -n parameter for zebra is taken
into account.
Also, when a extra parameter is given for calling zebra, a check is done to
see if it is possible to run zebra daemon with that option. This is the
case for vrfwnetns option. If not available, an error message is sent
back.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>