Add passing of config to plugin constructor.

Backwards-compatible API change. Needs the minor version of
PythonCollector bumped when it is merged. Currently targeting 1.3.
All collect method to return defer.succeed(None).

Backwards-compatible API change. Will require bumping the ZenPack's
minor version when it's merged. Currently targeting 1.3.
Allow ds_dp syntax for data['values'][c] keys.

Allows for multiple datasources with datapoints by the same name. This
is a backwards-compatible API change that will require bumping the
ZenPack's minor version when it's merged. Currently targeting 1.3.