PM-71: Robust ansible

Context

• Project README link: It's in the review!
• Task details: https://its.cern.ch/jira/browse/PM-71
• See https://gitlab.cern.ch/mpe-ms/pm/pm-backend/merge_requests/251 for the inventories, script, etc...

What was done

• Uninstall task added, and called when re-deploying an application
• Version is now stored in a text file, so there is no reliance on the directory path

QA

• Classic "It's Ansible, we don't know how to test it" statement

Review objectives

• assessment of code functionality
• assessment of code quality
• knowledge sharing