Remove lib/tempest errexit handling

lib/tempest had code to set -o errexit and restore the setting after
configure_tempest had run. This code didn't work, but it is no longer
necessary since we set errexit for all of devstack (see change 88521;
Change-Id Ia477e04567046a7fcdedf699423165bc577b72ad).

This changeset removes the errexit handling code from lib/tempest.

Change-Id: I2b0bc3c55a5d9bbb688f1e2e8e0fe133830a4172
Merge "Remove lib/tempest errexit handling"