zdtm: Test --external veth option (v5)

Inspired by Tycho's macvlan test, here's the same thing for
--external veth option. In master we still have the --veth-pair
one, but the plan is to move this all under the --external opt.

v2:
* Travis doesn't have /usr/bin/sed
* Added .checkskip hook for older environments

v3:
* Delete bridge hanging around after previous flavor
* Wait for host veth end to die after dump

v4:
* Get the pid of task to move veth into from .pid.inprogress file

v5:
* Wait for host veth end to die after test stop too :\

Travised-by: https://travis-ci.org/xemul/criu/builds/170726663


Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>