First steps for entry point plugin conf file loading

Currently, the platform doesn't have a way to automatically load a config file for a plugin that's installed through entry points. This PR doesn't fix that.

But it does take steps. The proposal is to have the conf files installed per plugin in /etc/pulp/server/plugins.conf.d. The server can read them if found and collate to the plugin.

I need this for the new yum importer but don't have time to do the full server implementation. So this PR is to have the directory in place and correctly owned by the platform. For now, the new yum importer will load the file itself from this location until the platform can do it.