Replace static dict mapping version to packages with a method (LP: #848932)

Previously, there was a 'ruby_packages' dictionary that mapped the
ruby version (1.8, 1.9, 1.9.1) to a list of packages that would need
to be installed to get a functional gems.

This replaces that with a method that is more likely to support future
versions without requiring updates to cloud-init.

It is not identical output as before.  The changes are:
 * do not include 'ruby' in the case of 1.8, but rather 'ruby1.8'
   This is because the default could change, and 'ruby' would depend
   on a different default version.
 * do not explicitly list 'libruby-<version>' as that is a dependenency
   of 'ruby<version>'
 * End result is for any 'version' != 1.8, you'll get the following installed
    ruby<version>
    ruby<version>-dev

LP: #848932

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