Add mpcut.sh for milestone-proposed branch cutting

Add an mpcut.sh script (similar to the mpdelivery.sh script) which
handles all steps required at milestone-proposed branch cutting
time in the release process:

- Creating milestone-proposed branch in gerrit
- Wait for tarball creation (calls wait_for_tarball.py)
- Set all FixCommitted bugs to FixReleased (calls process_bugs.py)

Change-Id: I417ad6247501afd7c18fdc989ae7b561c2c32814