introduce NEXTCLOUD_CONFIG_DIR env variable (see #300)

nextcloud by default uses the `/config/` directory in the source/application tree for its config file(s).
with this commit that directory can be overridden by the `NEXTCLOUD_CONFIG_DIR` environment variable.

in uwsgi, you would use the option `--env "NEXTCLOUD_CONFIG_DIR=/tmp/nx-config/"`
in apache `SetENV …`
and the cli command can be run with: `NEXTCLOUD_CONFIG_DIR=/tmp/nx-config ./occ` (or just use `export` once in the
shell).

NEXTCLOUD_CONFIG_DIR can be supplied with or without the trailing slash (`/`), but in all cases `$configDir` will have
it automatically added if needed.

The other changes are several occurrences of `OC::$SERVERROOT . '/config'` to `OC::$configDir`.