Fixes for Varnishkafka gmond module

* Make the module write / update the pyconf file in place.
* If the pyconf file already exists, print a diff of the update.
* If '--dry-run' was specified, print the diff, but don't update/write.
* Set exit status of 1 if nothing to do, 0 if update was successful (or
  if --dry-run and everything looked sane).

Result: the exec resource that updates the file will run onlyif a dry-run
invocation indicates that the pyconf file needs to be generated/updated.

Change-Id: I0b262c31a004fef90c9305389bb6103a7321d563