tests: prevent hook validation decorator from leaving scripts installed

It fixes a bug in the hook validation decorator, that left tests
scripts installed into the system. Now they're properly removed
right after the tests.

Adds rmTree function to vdsm.utils.

Change-Id: I138c3eab77ea6d35d6a997049940923026e06d96
Signed-off-by: Miguel Angel Ajo <miguelangel@ajo.es>
Reviewed-on: http://gerrit.ovirt.org/20310
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>