(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.
Yeah, this is a good question.  We ended up doing something similar in the irb and ruby subcommands as well, https://github.com/puppetlabs/puppet-server/blob/master/src/clj/puppetlabs/puppetserver/cli/irb.clj#L16-L17, but I'm not sure if that was the best choice because of possible pollution, as you mentioned.

In both the gem and irb cases -- not sure about the ruby subcommand -- there was code being executed that needed the PATH environment variable to be defined.  Rather than just assuming the PATH from the user's environment, it may be better to find out what it is that the code needs to resolve from the PATH and setup the environment variable in code with just that minimal set and appropriate for JRuby execution.

If it turns out that PATH needs include the JRuby bin directory, it would seem better to put that in the PATH programmatically like we do for GEM_HOME.  Could call `.getHomeDirectory` on the `ScriptingContainer` and append "/bin/" to it.

If it turns out that we need to preserve some of the default environment variables on the container, I think it would be better to just get these via a call to `.getEnvironment` on the `ScriptingContainer` rather than through `.getEnvironment` on a new `RubyInstanceConfig`.