Tensorflowonspark

- removed `tensorflow` as a dependency, in order to support other variants like `tensorflow-gpu` or `tf-nightly`.
- allow use of `evaluator` node type in cluster (thanks to bbshetty)
- refactored cluster template generation.
- updated wide-deep example to use models/official code.
- restore termination of feed in mnist/spark example.
- updated sample notebook instructions.
- updated to use Spark 2.3.3.

- Set TF_CONFIG for "chief" clusters (required by DistributionStrategy APIs)
- Fix GPU allocation for multi-gpu nodes
- Updated examples for MNIST
- Updated Hadoop and Spark dependency versions

- Added `util.single_node_env()`, which can be used to initialize the environment (HDFS compatibility + GPU allocation) for running a single-node instance of TensorFlow on the Spark driver.
- Added an example of parallelized inferencing from a pre-trained SavedModel.

- More deterministic GPU allocation for multi-GPU nodes.
- Added `timeout` argument to `TFCluster.shutdown()` (default is 3 days). This is intended to shutdown the Spark application in the event that any of the TF nodes hang for any reason. Set to -1 to disable timeout.
- Added ability to start reservation server on a specific port (contributed by AvihayTsayeg).
- Updated pipeline API for latest TF APIs (contributed by AvihayTsayeg)
- Added unit test for `tf.SparseTensor` support.
- Updated examples to latest TF APIs (including workaround for https://github.com/tensorflow/tensorflow/issues/21745).
- Updated Spark version dependency for Scala Inferencing API.
- Added `__version__` to module.

- Travis CI integration for Python documentation and Scala Inferencing API builds.
- Added `sys.path` to tensorboard search path.

- Only set TF_CONFIG environment variable if cluster_spec has a "master", i.e. when using `tf.estimator`.
- Updated `mnist/keras/mnist_mlp_estimator.py` with example of distributed/parallel inferencing via `estimator.predict`.
- Added optional `feed_timeout` argument to `TFCluster.train()` for InputMode.SPARK.
- Added optional `grace_secs` argument to `TFCluster.shutdown()`.
- Workaround for firewall proxy issue with `get_ip_address` (contributed by viplav).
- Add support for all Hadoop-compatible File System schemes (contributed by vishnu2kmohan).
- Added error messages to `assert` statements.
- Initial Travis CI integration.