configurable hostname for published address strategy

Allow configuration of published hostname in activemq.xml. Currently, there is no way to configure the hostname for the published address strategy in activemq-osgi; a custom PublishedAddressStrategy is required to return a different hostname (e.g. for clients that connect to the broker via a 'public' hostname, not the internal/real hostname).

Example/Custom PublishedAddressStrategy: https://github.com/sjhiggs/amq-pub-addr-policy-custom