(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 did not realize that the other subcommands do the same thing.  In that case, I think this patch is fine, and we should probably come back around and clean this up (as you suggest, @camlow325) in all of the subcommands at once.