Fixes config schema for kryo TOPOLOGY_KRYO_REGISTER

Corrects `TOPOLOGY_KRYO_REGISTER_SCHEMA` by adding a new validator for Strings or Maps of Strings, and tests for the new validator.

This should fix integration-test case failures:

`test-component-specific-config`
`test-ack-branching`
`test-acking-self-anchor`

exposed by #11 and found in https://github.com/apache/incubator-storm/pull/14#issuecomment-31796033


Thanks to @revans2 for this work.