Add executor-specific setup time support

This will allow us to make the setup time TTL for k8s-based builds much lower (on the order of a minute), which means faster timeouts and fallbacks (which is a better user experience).

buildman/manager/executor.py

@@ -55,6 +55,13 @@ class BuilderExecutor(object):
     """ Name returns the unique name for this executor. """
     return self.executor_config.get('NAME') or self.__class__.__name__
 
+  @property
+  def setup_time(self):
+    """ Returns the amount of time (in seconds) to wait for the execution to start for the build.
+        If None, the manager's default will be used.
+    """
+    return self.executor_config.get('SETUP_TIME')
+
   @coroutine
   def start_builder(self, realm, token, build_uuid):
     """ Create a builder with the specified config. Returns a unique id which can be used to manage