Add support for /etc/swift/proxy-server/proxy-server.conf.d 'format'

[ghost]

This will cause issues down the road, due to the generated ... it will only include the ONE proxy file you chose to include (unlike object conf files we don't support multiple proxy file) and that'll lead to an issue like:
Jul 10 05:37:21 saio-roumani object-6040: No section 'proxy-server' (prefixed by 'app' or 'application' or 'composite' or 'composit' or 'pipeline' or 'filter-app') found in config /etc/swift/storlet-proxy-server.conf

When we try to use the internal client to fetch the storlet and dependencies.

I suggest documenting this and maybe saying to fix you need to manually merge all the files in the to the storlet proxy conf after !