(maint) Preserve :__commandline_args__ if provided in conf_opts

Oops, made the mistake of rudely ripping :__commandline_args__ from
with_puppet_running_on's conf_opts when I first added this bit.  This
should preserve the :__commandline_args__ for future callers while still
keeping it out of the puppet.conf creation step.
Merge pull request #169 from jpartlow/dont_destroy_callers_opts

(maint) Preserve :__commandline_args__ if provided in conf_opts