Unify the way how frontend checks whether backend is online in case of single vs. cluster mode

[tomas-knap]

Please unify the way how frontend checks whether backend is online. So even in case of single backend, RMI is not used for checking whether backend is online, but DB is checked instead. (as in cluster mode)

Expected changes for frontend:

• No need to define whether frontend works in single/cluster mode using backend.cluster.mode
• status update whether backend is online will not be instant, but will have certain delay based on the settings of the param backend.alive.limit (I suggest by default 10s)
• when pipeline is run/debug from frontend, it may take up to 2s before certain backend really runs the pipeline

Expected changes for backend:

• backend.id is mandatory also in cluster mode
• backend.cluster.mode property not needed
  This update may be also solved in a separate pull request (if not doable before vacation)

[tomas-knap]

Follow-up of #482