don't corrupt grenade exit value
don't corrupt grenade exit value

The grenade exit value was getting masked in the exit handler
by doing something else before we captured it. Fix that, which
should make gate fails a little easier to resolve.

Change-Id: I61a950bf25319945292a36a70b7508839014d59f

dtroyer/grenade,dtroyer/grenade,openstack-dev/grenade,openstack-dev/grenade