Refactor PodStatus to PodCondition in internal API for v1beta3

Change PodStatus (string) to PodCondition (string) to make space for new PodStatus (struct) in v1beta3 API.

Change the constant values for Pod statuses to match v1beta3 API.
A side effect of throwing an error in the default case is validation of the constant values.  Some tests had arbitrary strings for those values, but now only valid ones can be used.