<oo-install> bugfixes

- Fix setting of OPENSHIFT_VERSION in Rakefile
- Prompt for district/node questions when adding the node role to an
  existing host.
- prevent nil from being added to the list of gear sizes
- only ask district/node questions if context is :ose
- Set ose-2.1 as the default for the ose context
- Update origin contexts to use 2.1 openshift.sh
<oo-install> update for github raw URL change