Allow list_deliverables to filter output to list only deliverables
that have not been released since a given date (--unreleased-since).
This will be useful to compile list of libraries to autorelease at
milestones, or check for cycle-with-intermediary services that may
need a refresh release.
Additionally, if --show-dates is provided, the dates for last releases
(or all releases if -a option is provided) will be retrieved (and
displayed if in verbose mode).
Retrieving release dates is done through querying opendev's API for
tag references and looking at committer dates, so it requires Internet
access and increases command run time. It is only enabled if
--show_dates or --unreleased-since are enabled.
Task: 35684
Change-Id: I00aff6703e85a00572edb363973c92b883a79456
845e10c092