This introduces a new option to the gnocchi collector: "re_aggregation_method".
It allows to specify an aggregation method that's different from the
retrieved aggregate type for dynamic aggregation.
Work items:
* Added a "re_aggregation_method" to the gnocchi collector
* Updated the documentation
* Added some unit tests
Change-Id: Id176c99a8cfc7761ba2a67c89a5521d23505ecb5
8a0f80ad91