Travis: calculate the number of layers for images to squash

[jkremser]

• 1 commit is about smarter squashing, where the number of layers to squash is calculated based on the docker history command output (instead of hard-coded 22 value as it was)
• and the second commit is more a refactoring where I moved those lines from https://github.com/radanalyticsio/openshift-spark/blob/e8f3c489421a1b36fcee0d0af694a43d8baaa134/.travis.yml#L12:L16 to its own bash script and not running it in the deploy job when it wasn't necessary ... there is no need to install openshift if we wouldn't be pushing the images

[jkremser]

unfortunately, the deploy job will be skipped because this is a PR and it may fail again when it's run against the master commit

I've run the Travis on my fork on master https://travis-ci.org/Jiri-Kremser/openshift-spark/builds/539247845

it will fail, because the credentials for the image registries are not being set there, but if it's the only issue, we should be fine

[jkremser]

tests on my fork look good, :ship:it?