Add bin/testenv-ctl stop_jobs/start_jobs script

Adds a new bin/testenv-ctl script to help stop/start job processing
for all test-env workers on a node.

Running 'testenv-ctl stop_jobs' sends a SIGUSR2 to all testenv-worker
processes so that they stop processing any new jobs (current
jobs will finish).

Running 'testenv-ctl start_jobs' sends a SIGUSR1 to all testenv-worker
processes so that they resume job processing again.

This should allow us to upgrade testenv-worker nodes for the CI
overcloud without disrupting the job queue.

Change-Id: I8535e13c2fdd1a305feaa1ce0bfe3ef3eb5db685
Merge "Add bin/testenv-ctl stop_jobs/start_jobs script"