Allow override of -couch_ini parameter

This is the continuation of commit 649b808 "Allow override of
`-args_file` and `-config` parameters (apache#1095)".

This allows setting the `-couch_ini` files location if needed, using
`COUCHDB_INI_FILES` env var:
- If unset (the default), `-couch_ini` will have an empty list and
  CouchDB will start as usual.
- If set (to a file, or list of files like
  `COUCHDB_INI_FILES=/chroot/etc/couchdb/default.ini
  /chroot/etc/couchdb/default.d ...`) it will allow running CouchDB with
  different settings. A useful case is being able to run multiple
  CouchDB servers on the same machine.

rel/files/couchdb.in

@@ -31,8 +31,10 @@ export COUCHDB_QUERY_SERVER_COFFEESCRIPT="{{prefix}}/bin/couchjs {{prefix}}/shar
 export COUCHDB_FAUXTON_DOCROOT={{fauxton_root}}
 
 ARGS_FILE="${COUCHDB_ARGS_FILE:-$ROOTDIR/etc/vm.args}"
+INI_FILES=${COUCHDB_INI_FILES:-}
 SYSCONFIG_FILE="${COUCHDB_SYSCONFIG_FILE:-$ROOTDIR/releases/$APP_VSN/sys.config}"
 
 exec "$BINDIR/erlexec" -boot "$ROOTDIR/releases/$APP_VSN/couchdb" \
      -args_file "${ARGS_FILE}" \
+     -couch_ini ${INI_FILES} \
      -config "${SYSCONFIG_FILE}" "$@"