Set virtualHost on StompBrokerRelayRegistration

Prior to this commit, one couldn't set the virtualHost property on
StompBrokerRelayMessageHandler via JavaConfig, since
StompBrokerRelayRegistration's API didn't offer that possibility.

This commit adds a new method in StompBrokerRelayRegistration's fluent
API to set the virtualHost used by StompBrokerRelayMessageHandler.
Note: this property is already configurable via xml config.

Issue: SPR-11433