Add support for a centos-based distro

[jhfrontz]

This updates gbuild to be compatible with Centos's package manager (yum). It also adds a few robustness improvements (redirecting cd executions to /dev/null, preventing any shell-prompt-related output from causing confusion).

I've successfully tested/used this with Docker running against an image created with $ bin/make-base-vm --docker --distro centos --suite 7 --arch amd64.

[jhfrontz]

@achow101 can you review this?

[jhfrontz]

Note -- I somehow forgot earlier to include the corresponding changes in make-base-vm in my previous commit. I just updated/squashed.

[achow101]

utACK 25630b7acca5db2d2c033d5d21240fc2257c2396

I don't have a CentOS system and am not particularly familiar with them, but this looks correct. At the very least, the debian/ubuntu parts seem to be exactly the same as before.

[jhfrontz]

Updated to improve compatibility with newer versions of docker.

[wtogami]

needs rebase