add minitaur environment randomizer, enable it by default

this improves PPO training ( more stable)

examples/pybullet/gym/pybullet_envs/bullet/env_randomizer_base.py

@@ -0,0 +1,25 @@
+"""Abstract base class for environment randomizer."""
+
+import abc
+
+
+class EnvRandomizerBase(object):
+  """Abstract base class for environment randomizer.
+
+  An EnvRandomizer is called in environment.reset(). It will
+  randomize physical parameters of the objects in the simulation.
+  The physical parameters will be fixed for that episode and be
+  randomized again in the next environment.reset().
+  """
+
+  __metaclass__ = abc.ABCMeta
+
+  @abc.abstractmethod
+  def randomize_env(self, env):
+    """Randomize the simulated_objects in the environment.
+
+    Args:
+      env: The environment to be randomized.
+    """
+    pass
+