Add BrooklynNode.PORT_MAPPER

- For if working with DNAT, the port we ask for might not be the same
  as the port we use in the brooklyn web-console.
Expose BindDnsServer.getAddressMappings as effector
BindDnsServer: fix comment format in domain.zone

- comments are semi-colon rather than c-style
  (at least on the CentOS 6.4 VM I'm testing with!)
BindDnsServer: don't extend JavaSoftwareProcessSshDriver

- instead extend AbstractSoftwareProcessSshDriver because this is
  not using java
BindDnsServer: fix race for adding existing entities