Clean up gerrit push change script

Removed broken code that removed leading/trailing whitespace from
variables. Use lf-acitvate-venv() to instll openstack. Enabled 'set -euf
pipefail' and updated code to handle errors. Updated out of date
comments.

ISSUE: RELENG-2633
Change-Id: I76d2bf9ed9f7f26303f56dd98ebbd4b458d0d54f
Signed-off-by: Tim Johnson <tijohnson@linuxfoundation.org>