Prepare Marconi to support oslo's config.generator

This patch removes the options_iter function and prepares marconi's code
base to play nice with oslo's config.generator. The replacement of lists
with tuples is necessary to avoid oslo's config.generator to parse the
globally defined options. In Marconi's case, we want it to use the
registered entry_points. (This is a workaround).

Change-Id: I56995fe16cd0b6e092b522dac3c32e4ec75bb269