docker: make docker-exec the default execution driver

Docker 1.13.1 removed the ExecutionDriver from the ``docker info``
return data. This causes all attempts to run commands in containers to
fall back to the old lxc-attach driver, which is incompatible with newer
docker releases.

salt/modules/dockerng.py

@@ -203,6 +203,14 @@ Functions
 Executing Commands Within a Running Container
 ---------------------------------------------
 
+.. note::
+    With the release of Docker 1.13.1, the Execution Driver has been removed.
+    Starting in versions 2016.3.6, 2016.11.4, and Nitrogen, Salt defaults to
+    using ``docker exec`` to run commands in containers, however for older Salt
+    releases it will be necessary to set the ``docker.exec_driver`` config
+    option to either ``docker-exec`` or ``nsenter`` for Docker versions 1.13.1
+    and newer.
+
 Multiple methods exist for executing commands within Docker containers:
 
 - lxc-attach_: Default for older versions of docker
@@ -843,10 +851,12 @@ def _get_exec_driver():
             __context__[contextkey] = from_config
             return from_config
 
-        # For old versions of docker, lxc was the only supported driver.
-        # This is a sane default.
-        driver = info().get('ExecutionDriver', 'lxc-')
-        if driver.startswith('lxc-'):
+        # The execution driver was removed in Docker 1.13.1, docker-exec is now
+        # the default.
+        driver = info().get('ExecutionDriver', 'docker-exec')
+        if driver == 'docker-exec':
+            __context__[contextkey] = driver
+        elif driver.startswith('lxc-'):
             __context__[contextkey] = 'lxc-attach'
         elif driver.startswith('native-') and HAS_NSENTER:
             __context__[contextkey] = 'nsenter'
@@ -5510,9 +5520,7 @@ def get_client_args():
     .. versionadded:: 2016.3.6,2016.11.4,Nitrogen
 
     Returns the args for docker-py's `low-level API`_, organized by container
-    config, host config, and networking config. This is designed for use by the
-    :mod:`docker states <salt.states.docker>` to more gracefully handle API
-    changes.
+    config, host config, and networking config.
 
     .. _`low-level API`: http://docker-py.readthedocs.io/en/stable/api.html