copy artifacts from base tempest to target
copy artifacts from base tempest to target

Instead of recreating the tox and testrepository environments from
scratch, copy the contents of the base runs into the target
runs. Tempest tox creation takes about 2 minutes, so this should drop
2 minutes from all the grenade runs.

Change-Id: I4dfc33183ab69c4984a0d5bd234a4b767ee68228

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