Parametrize redis' settings needed for Wikimetrics

Wikimetrics is currently using a custom redis configuration template,
and we want to bring Wikimetrics' requirements back to the redis
module. Wikimetrics needs to be able to
* specify the dbfilename, as this file gets backed up for Wikimetrics,
* set custom save points, and
* needs to be able to have redis stop writing on errors for background
  writes.

We add options to the class that allow to tweak the redis
configuration accordingly.

While some of those could get set through the redis_options parameter,
that would make the instantiated template less straight forward, as
settings from the top of the configuration would get overridden at the
bottom and hence might go unnoticed when debugging. Also, setting
three separate "save" values, would require to have the key "save"
multiple times in the dictionary, which is currently not supported.
Hence, we add the needed options as separate parameters to the redis
class.

Bug: 66911
Change-Id: I7a0a226053d562f879f188d1e42ff0b71b352c0a