Code review comment for ~morphis/snappy-hwe-snaps/+git/build-scripts:f/release-from-stable

@Jim: https://trello.com/c/dVtwAPcV is the example in this case. master has evolved much further already with a lot new features and we need to bring a single change to caracalla as a new release of the network-manager snap. This single change was cherry-picked to stable with https://code.launchpad.net/~morphis/snappy-hwe-snaps/+git/network-manager/+merge/320352 and now we need to create a new release on top of what is in stable. This is what the current implementation doesn't allow but we change with this MP. With this we will

 - not merge master into stable
 - tag HEAD of stable with the new VERSION supplied by the user
 - push stable and trigger -publish build