refs #7970 - set default Ruby to 1.8 on Ubuntu 12.04

Ensures other applications, including hammer, work as 1.8's the expected
default version.

Needs https://github.com/theforeman/foreman-packaging/pull/398

Needs https://github.com/adrienthebo/puppet-alternatives/pull/17, so https://github.com/theforeman/foreman-installer/pull/129 updates foreman-installer.
I think I might drop the path attr, and just set mode.  This way it'll simply not automatically be set to 1.9 during installation, but won't force the value if the user decides to change it.