Bug 1066323 - [mozlog] Mozlog shouldn't propagate unremarkable defaults to the raw log r=jgraham

Currently, some of the raw JSON logs for mochitest and marionette, et al, include
empty dictionaries, None values and other unremarkable values that are marked
as optional. This fix aims to remove these unnecessary items from being
passed to the raw log.

A method has been added to the log_actions class which removes defaults if they
are marked as optional and the value is included in the default list. This is
called on the kwargs returned by the convert_known method, before being
propagated to the log_raw method for StructuredLogger.

Differential Revision: https://phabricator.services.mozilla.com/D25081