Fix logging in latest version of gcp

[ankurcha]

As per
https://github.com/GoogleCloudPlatform/fluent-plugin-google-cloud/blob/master/lib/fluent/plugin/out_google_cloud.rb#L1175

If message is present in the payload it's interpreted as a textPayload which is not what we want, it should be named something else, this hack gets around this.

[plelevier]

Hi ankurcha,

First, thanks for your work!

Why did you rollback this change in 1.1.6?

I've not tested yet 1.1.5 but after upgrading from 1.1.3 to 1.1.6, I still get GKE logs interpreted as a textPayload (The problem is on a recently upgraded kubernetes cluster. The appender still works fine with our old cluster)

[plelevier]

Ok I found out myself this issue on the google side : kubernetes/kubernetes#48108

So it seems it will be fixed on the next kubernetes version, until then we should not be using kubernetes 1.6.6 and stay with 1.6.4 (as described here : https://cloud.google.com/container-engine/release-notes )