Make force optional in app update

[shoenig]

Right now executing 'app update [id] [json]' will force an update. The Marathon API makes this optional to avoid bad things from happening, so we should provide this as an option as well.

[dontrebootme]

Along this line, we might want to also make the group updates optionally forced as well. If there is part of a group that is broken, you have to kill that deployment first.