Use branch for charms

When merging changes for charms in a development branch (ie: not v10.0, etc),
we want to publish it to latest/edge/branch-name so that it can be kept
separate from the other branches of development

Change-Id: I8f2ec65057d2029d7cda83b9b607a0474a801455
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
(cherry picked from commit 27da38deb3b184608a6d66ab6bc476506d8d7301)