Add spec2bp.py script

Add script to automatically set blueprint fields for an accepted spec.
Point it to a file in a -specs git repository, specify the target
milestone and off you go. By default, sets priority to Low:

Example:
spec2bp.py ../oslo-specs/specs/juno/graduate-oslo-i18n.rst juno-3

Change-Id: Ie5118e29deea1b455623117828057fca4cf69415
Merge "Add spec2bp.py script"