Update interval to 12h for sig-cli periodic e2e tests.
Update interval to 12h for sig-cli periodic e2e tests.

Per the [discussion in the 7/15/2020 SIG-CLI meeting](https://docs.google.com/document/d/1r0YElcXt6G5mOWxwZiXgGu_X6he3F--wKwg-9UBc29I/edit?pli=1#heading=h.himo1st0tqyy), the interval should be changed from 1h or 2h to 12h. This is a more reasonable frequency that still allows failures to be actionable if there is a failure.

jessfraz/test-infra,jessfraz/test-infra,dims/test-infra,pwittrock/test-infra,pwittrock/test-infra,fejta/test-infra,michelle192837/test-infra,BenTheElder/test-infra,cjwagner/test-infra,fejta/test-infra,pwittrock/test-infra,monopole/test-infra,cblecker/test-infra,BenTheElder/test-infra,monopole/test-infra,cblecker/test-infra,dims/test-infra,BenTheElder/test-infra,cjwagner/test-infra,brahmaroutu/test-infra,michelle192837/test-infra,monopole/test-infra,dims/test-infra,kubernetes/test-infra,michelle192837/test-infra,monopole/test-infra,fejta/test-infra,cjwagner/test-infra,jessfraz/test-infra,jessfraz/test-infra,BenTheElder/test-infra,kubernetes/test-infra,brahmaroutu/test-infra,michelle192837/test-infra,michelle192837/test-infra,dims/test-infra,brahmaroutu/test-infra,brahmaroutu/test-infra,kubernetes/test-infra,fejta/test-infra,jessfraz/test-infra,brahmaroutu/test-infra,cjwagner/test-infra,dims/test-infra,dims/test-infra,michelle192837/test-infra,pwittrock/test-infra,cblecker/test-infra,pwittrock/test-infra,jessfraz/test-infra,BenTheElder/test-infra,cblecker/test-infra,monopole/test-infra,kubernetes/test-infra,monopole/test-infra,cjwagner/test-infra,cjwagner/test-infra,fejta/test-infra,fejta/test-infra,kubernetes/test-infra,BenTheElder/test-infra,brahmaroutu/test-infra,kubernetes/test-infra,cblecker/test-infra,cblecker/test-infra