Release Script for Charms and Snaps

Performs a scan of the track and channels specified to promote
the snaps and the charms to the next track.

Change-Id: I1ed59f1a34fda93eba19c4477149d166f0998fc1
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
(cherry picked from commit 379b6d33)