Deprecate s2i-python2 in examples

[jklaise]

Any examples and docs using the Python2 wrapper image should be re-written for Python3 (related to #566), a grep through the repository for s2i-python returned the following:

 grep -nrw . -e 's2i-python2'
./examples/models/deep_mnist/deep_mnist.ipynb:648:    "!eval $(minikube docker-env) && s2i build . seldonio/seldon-core-s2i-python2:0.5.1 deep-mnist:0.1"
./doc/source/tutorials/openshift_s2i.md:92:s2i build <git-repo> seldonio/seldon-core-s2i-python2 <my-image-name>
./doc/source/tutorials/openshift_s2i.md:93:s2i build <src-folder> seldonio/seldon-core-s2i-python2 <my-image-name>
./doc/source/reference/images.md:11:| [Seldon Python 2 Wrapper for S2I](docs/wrappers/python.md) | [seldonio/seldon-core-s2i-python2](https://hub.docker.com/r/seldonio/seldon-core-s2i-python2/tags/) | 0.5.1 | deprecated |
./wrappers/s2i/python/s2i/bin/assemble:10:# If the 'seldon-core-s2i-python2' assemble script is executed with the '-h' flag, print the usage.
./wrappers/s2i/python/s2i/bin/usage:11:s2i build https://github.com/seldonio/seldon-core.git --context-dir=wrappers/s2i/python/test/model-template-app seldonio/seldon-core-s2i-python2 seldon-core-template-model
./wrappers/s2i/python/s2i/bin/usage:22:s2i build https://github.com/seldonio/seldon-core.git --context-dir=wrappers/s2i/python/test/router-template-app seldonio/seldon-core-s2i-python2 seldon-core-template-router
./wrappers/s2i/python/s2i/bin/usage:35:s2i build https://github.com/seldonio/seldon-core.git --context-dir=wrappers/s2i/python/test/transformer-template-app seldonio/seldon-core-s2i-python2 seldon-core-template-transformer
./wrappers/s2i/python/test_fbs/run_test_python2:1:s2i build . seldonio/seldon-core-s2i-python2:0.4 test-wrapper-py2:0.1