Add retry loop for getting dhcp port

* There is a possible condition (exposed by test_network_basic_ops) where
  the subnet will not yet have a dhcp port attached to it, but an attempt
  to add a metadata route using the dhcp port will happen. This is resolved
  by putting in a couple of retries to give time for the dhcp_agent to create
  a port.

Fixed MN-1028
Signed-off-by: Joe Mills <joe@midokura.com>