rest_api: Disable flaky test.

The test rest_api/channels/external_media/off-nominal
consistently fails on the GitHub CI, causing every
single test run to report failure. This test is being
disabled until such time as the issue with it is resolved.

Resolves: #28

tests/rest_api/channels/external_media/off-nominal/test-config.yaml

@@ -1,4 +1,5 @@
 testinfo:
+    skip: 'Unstable - issue 28'
     summary: Test External Media channel failure
     description: |
         Attempts to create an externalMedia channel with unsupported