Prepending "appscale-app" to app directories in /tmp

Prior to this commit, the AppScale tools would copy App Engine apps to /tmp/random-str, but if the upload failed for some reason this folder wouldn't be cleaned up. Since the string was random, users would have a hard time knowing if AppScale generated this file or if something else did, and it was hard to clean up those folders with a `rm -rf` command.

This pull request prepends `appscale-app` before the random string, so that the user can trivially run `rm -rf /tmp/appscale-app-*` if desired.