Switch to use cast method in dhcp_ready_on_ports method
When DHCP agent reports to the neutron-server which ports are
ready, it was using call() method from rpc client.
That caused blocking dhcp agent's dhcp_ready_ports_loop thread which
blocks to send info about other ready ports if there are any.
Method call() should be used when RPC caller returns a value to the
caller but that's not the case here. On neutron server side this
RPC method is only calling provisioning_complete() method to
finish provisioning of ports. And is not returning anything.
So to make sending dhcp ready ports to neutron-server much faster
this patch switch to use cast() method from rpc client.
This method don't block to wait for return value from RPC caller.
(cherry picked from commit