Assert requried circle CI envs in ci2gubernator (#5137)
Assert requried circle CI envs in ci2gubernator (#5137)

Automatic merge from submit-queue.

Assert requried circle CI envs in ci2gubernator

There has been cases where tests on circle failed when calling ci2gubernator because `CIRCLE_PR_NUMBER` unbound. This PR asserts the existence of the circle ci envs required by ci2gubernator and resort to no op if any of those is not defined.
istio/istio,smawson/istio,frankbu/istio,smawson/istio,geeknoid/istio,geeknoid/istio,istio/istio,istio/istio,geeknoid/istio,smawson/istio,geeknoid/istio,geeknoid/istio,istio/istio,geeknoid/istio,smawson/istio,frankbu/istio,istio/istio,smawson/istio,frankbu/istio,frankbu/istio,frankbu/istio,smawson/istio,frankbu/istio