Switch between OpenStack versions docs update.

After implementing possibility to switch between OpenStack
version flexibly using the origins configuration files -
the following sections were updated in the documentation:
* 'Configurations' section on the features page:
  - origins files location
* 'Configuration' section on the getting started page:
  - how to include origins file in command line
* 'How do I run a specific OpenStack milestone' section
  on the 'Questions and Answers' page:
  - origins configuration file parameters
  - specifying component branch/tag

Change-Id: I6ad2288b3f919531af02b553ed86d16536fc75e1