Parameterize machine names in edx-west playbooks

This is pulling into master a few changes we've made in the playbooks we use to drive our own Stanford instance.  Mostly want to keep master in sync so others can see what we're doing and our upcoming merges are cleaner.

@feanil -- can you take a look?  Just asking you since you've been looking at other of my changes, but this one isn't very exciting. 

Look at PR #854 for discussion with Joe and Jason about the over all feature.  Thanks.