cmd: add cmd.Context to command.Init

cmd: add cmd.Context to command.Init

Pass cmd.Context to command.Init. This is mostly unused, although most invocations of command.Run do not use their Context either.

https://code.launchpad.net/~dave-cheney/juju-core/102-cmd-init-context/+merge/124899

(do not edit description out of merge proposal)

[fwereade, 2012-09-18 12:54:47]

If I were implementing it I think I would have gone for something more like
c.Config.Open(ctx) inside Run, but I can't come up with a really convincing case
for that being *better*. (OK, it'd be less work, but the work's already done.)
So: LGTM.