tests: one Dockerfile per repository:tag

tests: one Dockerfile per repository:tag

There cannot be a common Dockerfile for all repository:tag combination
of a given operating system. The only way to customize a Dockerfile is
via variable substitution and it cannot conveniently address all
differences between versions.

Create one Dockerfile per operating system version instead. I.e. one
dockerfile for centos:7, one for centos:6 etc.

Signed-off-by: Loic Dachary <ldachary@redhat.com>