Remove PYTHONPATH requirement to execute run_slave.py.

slave/buildbot.tac

Index: slave/buildbot.tac
diff --git a/slave/buildbot.tac b/slave/buildbot.tac
index 3b540f95f48033b73781bf87036ad868c1747086..ffae3ce0a610a0a39a10c6fec4de1443442d4798 100644
--- a/slave/buildbot.tac
+++ b/slave/buildbot.tac
@@ -47,6 +47,12 @@ if password is None:
 
 if ActiveMaster is None:
   ActiveMaster = slave_utils.GetActiveMasterConfig()
+  if not ActiveMaster:
+    sys.stderr.write(
+        '\nCouldn\'t determine the slave type automatically.\n'
+        'Please update buildbot.tac to set ActiveMaster and slavename to the\n'
+        'relevant values.\n\n')
+    sys.exit(1)
 
 if host is None:
     host = ActiveMaster.master_host