ota_from_target_files: Add an option to not sign OTA packages

Sometimes it is useful to be able to tell ota_from_target_files
to not sign the output zip file. For instance, the private
release key may not be available when ota_from_target_files
is executed; similarly the release tools may not be available
or executable where the private key is stored.

This change adds an option, '--no_signing', to simply output the
unsigned OTA zip file, instead of spuriously signing it with the
test key even though the zip file would need to be re-signed later
with a different key.

Change-Id: I1f3c4dc8ffa35ce85478f848b147aff3d40fe283