Enable v1beta3 API via --runtime_config=api/v1beta3 flag

This exposes the proper v1beta3 API endpoint when the user specifies the --runtime_config=api/v1beta3 argument to the apiserver. v1beta3 is still considered experimental and subject to change.

--runtime_config is a map of string keys and values, that can be specified by providing

    --runtime_config=a=b,b=c,d,e

Only the key must be specified, the value can be omitted.

Enables v1beta3 in hack/local-up-cluster.sh and hack/test-cmd.sh

@bgrant0607

Will allow completion of remaining #1519 items
Sorry. Somehow added these comments in the wrong place. They were intended to be comments on:
```
versionHandler := apiserver.APIVersionHandler("v1beta1", "v1beta2", "v1beta3")
```