fixes #7970 - add and prefer /usr/bin/foreman-ruby symlink

Use foreman-ruby for bundle calls especially, so it allows the default Ruby
to vary separately from Foreman's.  Set foreman-ruby to ruby1.9.1 explicitly
on Ubuntu 12.04.

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Not finished, I've not copied this to all plugins + OSes, but easy to do so if it looks good.  @mmoll?

Scratch build at http://ci.theforeman.org/job/packaging_build_deb_coreproject/1285/label=debian,os=precise/ which worked well for me.  I was able to install the package, seeing foreman-rake and the bundle installs runs correctly while the default Ruby was 1.8.