Be less presumptuous about the GNU/Linux distribution we are being run
on, and do not try to install ruby >= 1.9.3 automatically.

refs #3363
* Default to using the auth.curoverse.com SSO server. Add instructions
in config.yml.example for using a local SSO server.

* Do not prepopulate the config.yml file generated by build.sh with
random values for _PW and _SECRET variables, let config.rb deal with
that at build time.

* Make config.rb a bit more robust in choosing those values for _PW and
_SECRET by not depending on any particular key order in the config Hash.

refs #3363
Update Docker install documentation:

* suggest use of build.sh to generate config.yml

* reflect now default use of Curoverse SSO server

refs #3363
Only the deepclean Make action should remove the skydns-image and skydock-image files.

refs #3363
Provide a bit more detail about build.sh in the documentation.

refs #3363
Merge branch '3363-docker-add-option-to-use-auth.curoverse.com'

refs #3363