Write metadata for ppc64; do not write product metadata if it has not changed

Fixes: https://bugs.launchpad.net/juju-core/+bug/1388493

This PR does 2 things:

1. If tools for ppc64el are found, metadata is also generated for ppc64 so that older versions of Juju which look for ppc64 can find tools.

2. When writing simplestreams product metadata for tools, if the generated metadata is the same as what's there, do not overwrite the file.