Fix environment for coverage runs.

This commit removes the DEVSTACK_GATE_TEMPEST_FULL enviromental variable
from the coverage periodic job definition. If this variable is set it
actually blocks coverage from being collected because of the if logic in
devstack-gate.

Change-Id: Ie3a692643bdcf5d5abce586b905e5f65e1f635dd
Reviewed-on: https://review.openstack.org/35399
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins