Integrated customizable bash prompt

Get a nice configurable shell prompt for all nodes... YEAH!
This is a followup pull request after discussion on https://github.com/crowbar/barclamp-deployer/pull/219
Actually it should be neither - it should create the file if it doesn't exist, and append the line to the file only if it isn't already there.  Currently it overwrites handwritten changes, which is bad because that file is specifically for handwritten changes.