[CORD-3003] Make XOS GUI optional (enabled by default)

Change-Id: I2b70ac628d7b5bf105de68895bb6cb0a22f93492
diff --git a/xos-core/requirements.yaml b/xos-core/requirements.yaml
new file mode 100644
index 0000000..f77c942
--- /dev/null
+++ b/xos-core/requirements.yaml
@@ -0,0 +1,21 @@
+---
+# Copyright 2017-present Open Networking Foundation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+dependencies:
+- name: xos-gui
+  version: 0.1.0
+  repository: file://../xos-gui
+  condition: xos-gui.enabled
+