Add new travis job for prebuilt images

[Tomcli]

This is an updated PR for #49

• Add a new Travis job to test DockerHub images and modify Travis to run 2 test jobs in parallel.
• Move the travis script commands under bin/travis_scripts.
• Closes Travis should test pulling external images for build #11

Note: The new Travis job only checks pre-built images working on master branch but not pending PRs because any pending PRs changes will not be reflected in the pre-built images yet. But you can find a sample prebuilt images run of this PR at https://travis-ci.org/IBM/FfDL/builds/399363365

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[animeshsingh]

This can probably be pulled from a config file? Makes it easier to extend.....

[Tomcli]

Sure, I can put these into a config file.

[animeshsingh]

Given that test environment is still minikube - what all we are not able to test here?

[Tomcli]

In theory we could, but putting everything into one single job will take longer time to finish. Also, it's easier to debug the 2 different environment if we run them in parallel.

[animeshsingh]

Added couple of comments