Add new version CentOS 7.6 dropping old versions

[junaruga]

It's same with another PR (https://github.com/multiarch/centos/pull/9) that was wrongly merged and canceled.

But I decided to share this PR, because I need your advises.

The situation is ...

As current exist 7.2.1511-${arch} 's image URLs do not exist except x86_64, those do not work. I updated to latest CentOS 7.6 (or 7.5) image files. Then, x86_64 and armhfp work. But other archs do not work. (docker build -t $repo:$version-iso iso works, but docker run -it --rm $repo:$version-iso ... does not work.)

Here is my repository's result. https://travis-ci.org/junaruga/centos/builds/531571388

Could you give me some advises?

Thank you.

---

Other notable features

• Install libguestfs-tools to use virt-tar-out command from xenial repo. We can not install it from bionic repo because of below error.

  The following packages have unmet dependencies: libguestfs-tools : Depends: libguestfs0 (= 1:1.36.13-1ubuntu3) but it is not going to be installed Depends: libguestfs-perl (= 1:1.36.13-1ubuntu3) but it is not going to be installed E: Unable to correct problems, you have held broken packages.

• Update qemu v3.1.0-3.

• Rafactoring mixed indent with spaces and tab, replacing tab to spaces.

• A bug of virt-tar-out command to convert iso file to tar file? After I asked it to the upstream project here. https://github.com/libguestfs/libguestfs/issues/37 , they recommended using guestfish instead of virt-tar-out for the issue case.

[junaruga]

@moul @moul-sudo I appreciate if you give me some advices for this PR when you have a time.

• armhfp container run (docker run) that is from "*.raw.xz" is succeeded.
• x86_64 containers run (docker run) that are from "*.qcow2" are succeeded.
• Other arch containers run (docker run) that are from "*.iso" are invalid. Failed to run docker run with the containers.

Detail: https://travis-ci.org/multiarch/centos/builds/531722254

[junaruga]

I rebased to pass the CI. I tried some ways to solve the invalid container images' issue. But as I could not solve it, I commented out those cases. I think this PR is still better than current master branch and CI's situation. Shall we merge this?

As a note, I removed Dockerfile files on the repository. As those are generated in update.sh, I think we do not have to keep those on the repository.

[junaruga]

@lafin could you review this PR?

[junaruga]

@lafin thanks for your review. I merged it now.