QPID-4809: change initial-config.json to using qpid.work_dir and qpid.home_dir config variables instead of QPID_WORK and QPID_HOME system properties

Enables us to default the former when QPID_WORK is not set and allows the latter to be set on a per-instance basis.

merged from trunk r1479176


git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.22@1479186 13f79535-47bb-0310-9956-ffa450edef68