Readiness probe: do not log tail errors

[barkbay]

In some cases it is not possible for the readiness script to log into stderr of PID 1.
It is the case for example if the container is started with the root user: uid of pid 1 is changed to 1000 by the startup script, hence the readiness script, which is running with the UID 0, is not able to write into /proc/1/fd/2

This is harmless but it generates the following message in the events:

  Warning  Unhealthy               36s                    kubelet, aks-nodepool1-41410888-1  Readiness probe failed: {"timestamp": "2020-01-07T14:55:14+0000", "message": "readiness probe failed", "curl_rc": "7"}
tee: /proc/1/fd/2: Permission denied

Since the log redirection is done as a best effort this PR skips any error message from tail, this prevents the error to appear in the events of the Pod.

Note that if the security context is correctly set (i.e. container is not running as root) this issue does not happen.

credits goes to @david-kow