Remove sleep command when retrieving OVS dp

OVS agent is a single thread module executed on a os-ken AppManager
context. os-ken uses, by default (and no other implementation is
available today [1]), "eventlet" threads. Those threads are scheduled
manually by the code itself; the context switch is done through
yielding. The easiest way to do this is by executing:
  eventlet.sleep()

If the assigned thread is not ready to take the GIL and do not yield
back the executor, other threads will starve and eventually will
timeout.

This patch removes the "sleep" command during the DP retrieval. This
will keep the executor on the current thread and will prevent the
execution timeouts, as seen in the bug related.

[1]1f751b2d7d/os_ken/lib/hub.py

Closes-Bug: #1861269

Change-Id: I19e1af1bda788ed970d30ab251e895f7daa11e39

neutron/plugins/ml2/drivers/openvswitch/agent/openflow/native/ofswitch.py

@@ -71,14 +71,13 @@ class OpenFlowSwitchMixin(object):
             dp = ofctl_api.get_datapath(self._app, dpid_int)
             if dp is not None:
                 break
-            # The switch has not established a connection to us.
-            # Wait for a little.
+            # The switch has not established a connection to us; retry again
+            # until timeout.
             if timeutils.now() > start_time + timeout_sec:
                 m = _("Switch connection timeout")
                 LOG.error(m)
                 # NOTE(yamamoto): use RuntimeError for compat with ovs_lib
                 raise RuntimeError(m)
-            eventlet.sleep(1)
         return dp
 
     def _send_msg(self, msg, reply_cls=None, reply_multi=False,