remove field 'key' in spec env from locust controller yaml

[uni-3]

Since "key" field seems not to be supported, I deleted it from the yaml files. I have confirmed that it worked with kubectl v1.8.0.

Thanks

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[uni-3]

I signed it

[googlebot]

CLAs look good, thanks!

[faizalkassamalisc]

I ran into this too. Any reason why this hasn't been merged?