Do not release the executor inside _check_child_processes

The method ``ProcessMonitor._check_child_processes`` was releasing
the thread executor inside a method that creates a lock for the resource
"_check_child_processes". Despite this resource is not used anywhere
else (at least for this instance), this could lead to a potential
deadlock.

The current implementation of ``lockutils.synchronized`` with the
default value "external=False" and "fair=False" is a
``threading.Lock()`` instance. The goal of this lock is, precisely, to
execute the code inside the locked code without any interruption and
then to be able to release the executor.

Closes-Bug: #2073743
Change-Id: I44c7a4ce81a67b86054832ac050cf5b465727adf
(cherry picked from commit baa57ab)

Author: ralonsoh

neutron/agent/linux/external_process.py

@@ -272,7 +272,6 @@ def _check_child_processes(self):
                            'resource_type': self._resource_type,
                            'uuid': service_id.uuid})
                 self._execute_action(service_id)
-            eventlet.sleep(0)
 
     def _periodic_checking_thread(self):
         while self._monitor_processes: