This commit moves the `nginx-proxy-http` service back to separate
`UPSTREAM_HOST`, `UPSTREAM_PORT`, and `UPSTREAM_PATH` variables, which
allows for more granular configuration, e.g. `proxy_redirect` patterns.
This commit unifies the `UPSTREAM_HOST` and `UPSTREAM_PORT` environment
variables to a new `UPSTREAM_ENDPOINT` variable, making additional
customizations (such as a `proxy_pass` to a sub-path) possible.
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.
