Added a try/except around a call to multiprocessing.cpu_count()

In some situations, the multiprocessing.cpu_count() function can raise
NotImplementedError. I.e. this can happen on Windows if the environment
will lack the 'NUMBER_OF_PROCESSORS' variable, or this variable will
contain a non-numeric character.

If this exception will occur, the worker will use a substitution value
of 1 CPU.

worker/buildbot_worker/base.py

@@ -315,7 +315,10 @@ def remote_getWorkerInfo(self):
                 if os.path.isfile(filename):
                     files[f] = open(filename, "r").read()
         if not self.numcpus:
-            self.numcpus = multiprocessing.cpu_count()
+            try:
+                self.numcpus = multiprocessing.cpu_count()
+            except NotImplementedError:
+                self.numcpus = 1
         files['environ'] = os.environ.copy()
         files['system'] = os.name
         files['basedir'] = self.basedir