(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.
This seems like it is equivalent to just using `System.getEnv()`.  If that's the case, it seems problematic .... in the main server-side code, we consciously avoid passing any unnecessary environment variables into the JRuby interpreters to avoid loading gems and things from any unexpected directories.  I wonder if we need to be concerned about the same thing here?