Add the option to disable master checks to the RPC client

Add the option to disable master checks to the RPC client

The option is propagated to the Transport class and allows to disable
checks for the master node, if the client is run on a different node on
purpose.

While at it, fix the documentation for the arguments of the constructors
of the classes.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>