[backend] msgbus: support for ssl + initial connection failover
    
    The 'host' and 'port' configuration options are obsoleted now in
    favor of 'hosts' array (reflects the stomppy implementation).  To
    be dropped.
    
    The 'auth' config option now allows user to specify client's
    key/crt and cacert bundle.
    
    Slightly modified the connection policy;  worker waits till the
    stomp connection is done before it continues with the assigned
    task.