executor: add extra kubectl port-forward debug and releasenotes

This change documents how the kubectl was not working and it adds
a log statement to help debug future issue.

Change-Id: Iaf6ca030365d9e4e768bca716568cb2d4289665d

releasenotes/notes/kubectl-port-forward-b6e5e8033c9b9ede.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    In some cases (such as when an older version of kubectl is present on the executor,
+    or the kubectl on the executor is from OpenShift), streaming logs from Kubernetes or
+    OpenShift pods did not work.
+    The log streaming system has been corrected to work with a wider range of kubectl
+    versions and error logging has been improved.

zuul/executor/server.py

@@ -355,6 +355,7 @@ class KubeFwd(object):
             self.port = m.group(1)
         else:
             try:
+                self.log.error("Could not find the forwarded port: %s", line)
                 fwd.kill()
             except Exception:
                 pass