backward compatibility of zoo.cfg

Before 3.5.x, users define zoo.cfg with "clientPort" parameter which is used to identify on which port the server is serving clients.

After upgrading to 3.5.x, the new format:

server.$id=$addr:$port1:$port2[:$role];[$cliAddr:]$cliPort

force users to define all the client ports on the entire ZK ensemble.

The goal of this issue is to preserve backward compatibility upgrading 3.4 to 3.5.
1. when a user defines an old-style config file, it should function the same as the old way – It should use clientPort variable and shouldn't create a dynamic file.
2. when a user with old-style config file tries to do reconfig relevant jobs, it should stop him and give out a warning.