Update centos-media for CentoOS 7.
Merge branch 'master' into centos7
Set the volume id when creating the ISO.

The bootloader tries to find the cdrom by volume id, and since we
weren't setting it, the system wasn't booting correctly.
Factor out commonalities between CentOS 6 & 7.

Additionally, added *.qcow2 to .gitignore; it's handy to be able to
create disk images when testing.