Fix/improve devstack snapshot builder
I recently tried to run the devstack snapshot builder Jenkins job for the first time in 3 months, and it failed with java.lang.InterruptedException early in provisioning (on two attempts). Fix this to work more reliably and run on a schedule (maybe weekly) so we find out earlier when it stops working.
Steps to Reproduce
Reason for Variance
User Impact Summary
We just need to create a special worker type for this job to get it running again. It used to run on the codejail worker, but this required that the codejail worker be a more expensive type, which was not optimal for testing.