Add bug-deferral feature to process_bugs.py

Add a new --milestone selection option in process_bugs.py to allow
selection of all open bugs from a given milestone.

That lets you defer bugs using:
./process_bugs.py glance --milestone juno-2 --settarget juno-3

Change-Id: I50f0081d49a491aaaa07c34f9dd9d9a608468e7f
Merge "Add bug-deferral feature to process_bugs.py"