Expose API to let project groups find blueprints matching a milestone tag

Per Danilo/Linaro. There is not currently an exposed API to search for blueprints at all from a project group.

IIUC, a change that would fit the current shape of the code would be something like this:

 - project groups expose the API to get the virtual milestones as filtered by tags.
 - these virtual tag-based milestones expose the "specifications" attribute.
 - Bob is your uncle.

Note that working on this would potentially add to tech debt: milestone tags work around other limitations, and building on them adds weight to a workaround.