dr: Add option to customize base image

Add the option to replace the base image to build the test container
image with quagga. This allows users to adjust the image reference
according to their environment (eg. in case a local image registry is
used).

Change-Id: I9ae3ef8bd5d42df801f9ee5b396d785ef42030c7
diff --git a/releasenotes/notes/dynamic-routing-base-image-12e76a6d85411a6d.yaml b/releasenotes/notes/dynamic-routing-base-image-12e76a6d85411a6d.yaml
new file mode 100644
index 0000000..303e095
--- /dev/null
+++ b/releasenotes/notes/dynamic-routing-base-image-12e76a6d85411a6d.yaml
@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    The new ``[dynamic_routing] base_image`` option has been added. This option
+    allows customizing the reference of the base container image used for
+    connectivity check in dynamic routing plugin tests.