ci-image-git-buildpackage doesn't support building for other suites than sid

Would it be possible to support building for other suites? (like stretch or stretch-backports) The suite could either be inferred from the branch name (assuming DEP14 layout) or from the changelog.

Originally filed at https://gitlab.com/eighthave/ci-image-git-buildpackage/issues/3 by @nicoonoclaste