Add more Variables/Settings to Configure

[arm4b]

Initial Minimum Viable implementation for installation of new packages was done in #42.

Real production use requires much more settings to configure.

• Ability to use unstable repo (?)
• Improve version/revision installation logic
• Ability to adjust st2.conf parameters Add vars to configure any 'st2.conf' and 'mistral.conf' settings #121
• nginx-related stuff (st2web)
  • Specify custom certificate Add an option to configure custom SSL certificate for st2web/nginx #133
• MongoDB
  • Allow adjusting st2 config [database] group settings via Ansible variables Add vars to configure any 'st2.conf' and 'mistral.conf' settings #121
  • MongoDB role setting changes should inject into st2 config.
  • Add ability to use external MongoDB server. Configure with Existing Mongo, RabbitMQ, PostgreSQL installations #17
• Much more to add [...]