This makes container builds more consistent, and therefore more
cacheable in relation to other containers.
In addition, certificates and keys are now copied to two separate
volumes, one containing all certificates (for use in services like
`nginx`, which may require unfettered access to certificates) and one
containing certificates only for a specific domain, for use in most
other cases.
This commit contains a fairly large diff for a fairly small change:
moving the `config/common` directory to `host/base` to better reflect
its intended use, and promoting `config/service` to the root directory.
These changes unlock some improvements in `coreos-home-server-update`
processes, which will (assuming `/etc/coreos-home-server/base` exists)
keep host-wide systemd services in sync in addition to service-specific
ones.
Changes have been make to the `Makefile` and a few other places where
`config/common` was referenced, but most of this work is renames that
are not intended to break compatibility with new or running servers.
