(SERVER-262) Fix `puppetserver gem env` subcommand

Without this patch the `puppetserver gem env` subcommand bombs out with the
following error:

    ERROR:  While executing gem ... (NoMethodError)
        undefined method `split' for nil:NilClass

The problem is a result of clearing the entire system environment resulting in
`ENV['PATH']` being `nil`.  This patch addresses the problem by associating
GEM_HOME and other variables into the system environment rather than clearing
it.
I went back and looked at the irb one briefly.  Part of the problem was that irb would need to spawn "sh" in a new process.  May not be a good idea for us to hardcode our best guesses at the locations of system paths in the setup of the PATH variable for these subcommands.  Although, it may still be good to inject the JRuby bin directory in front of the PATH that we give to the container so that anything there would supercede MRI counterparts that the user might typically have in their PATH.  (Hopefully, JRuby will honor a META-INF reference when evaluating the PATH environment variable).

I'm also okay with the patch on this PR as it stands although, again, I'd suggest moving this over to using `.getEnvironment` from the `ScriptingContainer` rather than the one from a new `RubyInstanceConfig`.