tests, tool: Suffix veth and dummy names with random strings

With much lower chance to collide we can simplify the process of creating
the virtual network devices.

Change-Id: I11a3d065ff8c4be74e07e9cd1fa80bf3f6b73cb0
Signed-off-by: Ido Barkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36323
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Ondřej Svoboda <osvoboda@redhat.com>