make gpu image Python 3 exclusive, closes #1324

[RafalSkolasinski]

Fix for #1324.

By symlinking pip -> pip3 and python -> python3 we ensure that all our s2i scripts that uses pip command will use pip3 for the seldon-core-s2i-python3-tf-gpu image.

[seldondev]

Tue Jan 14 14:28:07 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-1336/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1336 --build=1

[seldondev]

Tue Jan 14 14:28:13 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-1336/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1336 --build=2

[RafalSkolasinski]

/test integration

[seldondev]

Tue Jan 14 14:40:38 UTC 2020
The logs for [integration] [3] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-1336/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1336 --build=3

[axsaucedo]

Nice once @RafalSkolasinski
/approve
/lgtm

[seldondev]

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