Use rate instead of keep_prob in rnn_cell_impl.py
Use rate instead of keep_prob in rnn_cell_impl.py

While running rnn_cell_test found the following warning:
```
tensorflow/python/ops/rnn_cell_impl.py:1376: calling dropout (from tensorflow.python.ops.nn_ops) with keep_prob is deprecated and will be removed in a future version.
Instructions for updating:
Please use `rate` instead of `keep_prob`. Rate should be set to `rate = 1 - keep_prob`.
```

This fix replaces deprecated dropout with dropout_v2, and use rate instead
to fix the warning.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

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