Add functionality to inject files under nova.conf.d

[Mirek]

Add functionality to inject files under nova.conf.d as currently there is no easy way to overwrite anything except the [DEFAULTS] section. For example, I had a bug where I had to disable cache under [cache] section. At the moment the only option is to go and manually change nova.conf but then any Juju config change will re-create the config wiping your changes, only another option is to manually create nova.conf.d and put your changes there. However, again is manually and prone to any upgrades wiping it out again. It would be nice if you could pre-create conf.d directory and have the ability to do include-file://999-cache.conf. Similar to what you can do with telegraf charm for example.