E2E is broken by validations that enforce a plan is specified during provisioning

[jboyd01]

Jenkins E2E was disabled in early October: #1333 (comment) because of some sporadic failures we were encountering. There is an issue to address that (#1586)

As I was investigating re-enabling E2E tests I found that 0d741d5 introduced a regression that disallows the use of "default plans". We used to be able to provision a service instance without specifying a service plan if the referenced service class had exactly 1 plan - https://github.com/kubernetes-incubator/service-catalog/blob/master/plugin/pkg/admission/serviceplan/defaultserviceplan/admission.go#L51-L54 allows you to not specify a plan if the service only has a single plan. I.E., the E2E tests that are affected here are not specifying a plan on purpose to test and validate the defaultserviceplan admission controller.

I believe this should be fixed - we should allow the use of default service plans otherwise we introduce a regression in functionality.