Run travis builds for ppc64le

[Pensu]

This is in ref to #48 and an extension to the PR: #47. This PR allows the travis build to run for ppc64le env as well. Here are the changes done:

1. Add image name based on arch. For amd64 image names remains same, for ppc64le, add -ppc64le to the image name.
2. Add arch variable in travis.yaml to run the build on both amd64 and ppc64le archs and make go version same for all builds.
3. Disable shellcheck for ppc64le, as shellcheck is not available for the arch yet. (A issue has been opened: shellcheck for ppc64le koalaman/shellcheck#1767)
4. Download the correct dep binary for the given arch.

[k8s-ci-robot]

@Pensu: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[k8s-ci-robot]

Hi @Pensu. Thanks for your PR.

I'm waiting for a kubernetes-csi member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Pensu
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[Pensu]

Hi @msau42 @pohly @saad-ali, can you please help in reviewing this? Thanks.

[msau42]

/ok-to-test

[msau42]

We are in the process of moving away from travis to prow for image releasing. Do we know if prow supports cross builds?

[Pensu]

Closing in favor of #68