UX to upgrade a charm

[Gary Poster]

Hi Nick.  Something we don't have right now that we want is UX to 
upgrade a charm.  I filed this as bug 1100904.  For convenience, and 
possibly for discussion, here's what I wrote for that bug's description.

"""
If a charm is not local, we should give a control to upgrade it.

We can find out from the charm store whether a charm has a newer version 
available, and highlight the charm in the environment if we want to. 
Kapil says that the charm store needs an api to get current versions of 
a group of charms, as an optimization.

IIUC from my conversation with Kapil, even if we don't see a newer charm 
available, we should still allow a charm to be upgraded. This can 
trigger a hook to do make arbitrary changes. For the GUI charm itself, 
for instance, the charm might see if a newer release or newer branch 
were available, even though the charm didn't change. This understanding 
should be verified with Kapil.
"""

Thanks

Gary