Add a grid of tests for flannel

[justinsb]

Given the recent problems with flannel, which appears to be distro
specific, let's get some permutation testing going for all distros.

Currently every 8 hours, but once we get flannel figured out we should
drop this to every 24 hours (or even longer!)

[k8s-ci-robot]

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[rifelpet]

/lgtm

[hakman]

/lgtm

[k8s-ci-robot]

@justinsb: Updated the job-config configmap in namespace default at cluster default using the following files:

• key kops-periodics-grid.yaml using file config/jobs/kubernetes/kops/kops-periodics-grid.yaml

In response to this:

Given the recent problems with flannel, which appears to be distro
specific, let's get some permutation testing going for all distros.

Currently every 8 hours, but once we get flannel figured out we should
drop this to every 24 hours (or even longer!)

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