Update centos docker files

[nlgranger]

EDIT: updated to cover a broader issue

I think the centos docker file needs a little bit more love :-)

• Offer an image with manylinux2014 as the base image, this is the official baseline to build python packages with and would solve ROCm/tensorflow-upstream#567 and ROCm/tensorflow-upstream#522 without having the user change the OS (this is not an option for cluster users).
• Use devtoolset-8 instead of devtoolset-7 unless there is a good reason not-to
• I believe 'hip_samples', 'hip_hc', 'hip_base' at https://github.com/RadeonOpenCompute/ROCm-docker/blob/6e1b8265db83a51691040c201d79f12eaa879a7b/dev/Dockerfile-centos-7#L74 should be 'hip-samples', 'hip-hcc', 'hip-base'
• In the docker files, commands are sometimes split over several RUN lines. I think it generates spurious intermediate snapshots. For example, 'yum clean all' in ROCm-docker/dev/Dockerfile-centos-7 should be merged into the previous line and consecutive yum commands can be merged together.