Consistency with appcfg.sh / gae plugin

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The update command with appcfg.sh is

appcfg.sh update

but in gae plugin it is gaeUpload

The update backend with appcfg.sh is

appcfg.sh backends update

but in gae plugin it is gaeUpdateBackend

At the very least, gaeUpload should be deprecated and gaeUpdate should be used going forward.
The whole upload concept is a bit confusing as it isn't used anywhere else.

This gives consistency to the back end update command and appcfg.sh

(The Maven GAE Plugin uses mvn gae:update as well)