Prevent interactive mode over ssh on the host

Otherwise, the MOTD (and sometimes the error 'stdin: is not a
tty') will be printed to stdout.
Allow a quicker response if a location doesn't exist

Previously, the tools would ssh multiple times (num_retries)
before deciding that a location didn't exist.
Add the '--test' argument for appscale-add-keypair
Move overwrite check to its own function.