Add label sync cron job and add back label which has external dependency

[kunmingg]

Add config for label_sync cron job, and add "do-not-merge/work-in-progress" back to fix prow merge.
Details see: #102

[kunmingg]

/assign @DjangoPeng

[jlewi]

Can you add a comment about this being used by prow so not remove it.

[jlewi]

Can we remove in progress since we have /work-in progress?

[jlewi]

Should we change the status/approved label to approved and also add label lgtm? Those labels are used by the k8s-ci-bot. So status/approved is duplicative. I think by adding them here we can ensure consistent colors.

[jlewi]

Can you also add label: "area/usage" for issues related to reporting and metrics.

[kunmingg]

@jlewi config updated and tested.
Sample action on "kubeflow/community":

time="2018-04-10T10:38:25-07:00" level=info msg=migrate from="status/approved" repo=community to=approved
time="2018-04-10T10:38:25-07:00" level=info msg=migrate from="status/in progress" repo=community to="do-not-merge/work-in-progress"
time="2018-04-10T10:38:25-07:00" level=info msg=migrate from="status/lgtm" repo=community to=lgtm
time="2018-04-10T10:38:25-07:00" level=info msg=recolor color=2efca4 label=lgtm repo=community
time="2018-04-10T10:38:25-07:00" level=info msg=create label="area/usage" repo=community
time="2018-04-10T10:38:25-07:00" level=info msg=recolor color=03db12 label=approved repo=community

[jlewi]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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