Expose address of DC as runtime attributes on the HC

Currently there is no way to learn about the http management port of the DC from a slave's host.xml or runtime.

17:02:31] <pilhuhn> I am reading host.xml to find the DC, so that I can contact its http port for e.g. querying the /socket-binding-group if the one on the host is undefined
[17:02:41] <+bstansberry> I don't want it in the config, but I'm ok with adding it as a runtime attribute
[17:04:13] <pilhuhn> bstansberry fine with me when I can query the HC for the http port of the DC
[17:04:46] <+bstansberry> pilhuhn: the native API port should be a runtime attribute as well
[17:04:51] <+bstansberry> yes please
[17:05:04] <+bstansberry> what i mean by that is we should expose it as a runtime attribute
17:05:49] <+bstansberry> the config bit is an instruction to the HC, but we are going to add alternatives, e.g. a multicast address
[17:06:55] <+bstansberry> so using the config will not be a reliable source; runtime can be