Adds Hostnames for OSO Instances

Previously, the OSO broker/node instances would get spawned
using a combination of stack name, nova instance name and UUID.

This patch allows a user to specify a hostname for broker and
node instances.  It makes working with broker/node instances
much easier from a user's perspective.

Change-Id: Ib077acb257e5add6c36b4a913cc6752c716524a7