Prevent faulty localBroadcastAddress to be set

[dapc11]

When Cassandra has not yet started up and /metrics is called, then FBUtilities.getlocalBroadcastAddress is called and sets variables which causes faulty address to be set for the local broadcast address.

Using the same logic as FBUtilities, but not assigning any variable that Cassandra server itself is using elsewhere.

[dapc11]

This is a fix for when doing metrics scraping at an early stage of Cassandra startup. When calling FBUtillities.getlocalBroadcastAddress the field is set, resulting in that it's not assigned to the proper address when Cassandra has started up.

Related to #42, not fixing the stacktrace in log, but fixing so that StorageService can populate the broadcast address properly.