vagrant: refactor swift config script

This adds a util function for handling swift configuration.
vagrant: add some comments to swift config script
vagrant: add job chaining middleware to swift conf
vagrant: raise if pipeline target is not found

In `configure_swift.py`, before we were just append to the pipeline list
if the target wasn't found. We should be more conversative: if the
target isn't found, something has gone wrong.
vagrant/configure_swift: add some docstrings
vagrant: add better restart instructions to README
Merge pull request #123 from larsbutler/vagrant-job-chaining

Vagrant: job-chain middleware configuration