Use bytes for python3 friendly os.write

Bytes not str. Otherwise we get
TypeError: a bytes-like object is required, not 'str'
in the metadata proxy and it dies.

Closes-Bug: #1661303
Change-Id: If6b6f19130c965436a637a03a4cf72203e0786b0

neutron/agent/linux/daemon.py

@@ -136,7 +136,7 @@ class Pidfile(object):
 
     def write(self, pid):
         os.ftruncate(self.fd, 0)
-        os.write(self.fd, "%d" % pid)
+        os.write(self.fd, b"%d" % pid)
         os.fsync(self.fd)
 
     def read(self):

neutron/tests/unit/agent/linux/test_daemon.py

@@ -181,7 +181,7 @@ class TestPidfile(base.BaseTestCase):
 
         self.os.assert_has_calls([
             mock.call.ftruncate(FAKE_FD, 0),
-            mock.call.write(FAKE_FD, '34'),
+            mock.call.write(FAKE_FD, b'34'),
             mock.call.fsync(FAKE_FD)]
         )