Make jeepyb issue tracker agnostic

Jeepyb has scripts called by Gerrit hooks designed to update the corresponding bugs or blueprints associated to the change that Gerrit is handling at any time.

This allows a closed-loop process, where a developer can simply put in a commit description a string like 'Closes-Bug: <bug number> and Jeepyb will update the bug with useful data and change the bug status depending on the kind of the change event.

Jeepyb code is currently designed for Launchpad.
This implies we will lose the Jeepyb features when we switch over to Storyboard, and it makes difficult to adapt the Jeepyb niceties to other groups or organizations that use other issue trackers.

The intent of this blueprint is to lay out functional specifications for a pluggable issue tracker architecture in Jeepyb.
The functional specs should cover at the very least current Launchpad bug/blueprints workflows, as well as Storyboard workflows. We should also design it in a way that is easy to implement other issue trackers plugins, as this will allow consumption by downstream organizations.