Updates GCE channels to use ubuntu over COS

[geojaz]

I didn't realize we were defaulting to COS on GCE- this changes the default for >=1.17.0. I discovered this while researching #9832. When I tried to use calico on COS on GCE, I ran into the same filesystem mount issues we saw with coreos on #7931.
Changing to this image fixes everything and calico comes up properly :)

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[hakman]

Very nice. Ubuntu FTW!
/lgtm