Allow to unset environment setting

Added some logic to optionally remove the environment setting from the `puppet.conf` by setting `environment => undef`.
for clarity, it would be nice to use `if $environment == undef` instead of `if $environment != undef`.
the `!=` syntax is like a double-negative.

wrt `undef` semantics, @stephenrjohnson should clarify in README what `undef` should mean within context of configs. see thread at https://github.com/stephenrjohnson/puppetmodule/issues/63