Instead of referring to minor version of Centos, use the currently last
released one version 7.

/reviewed-on nexedi/slapos.package!30

There is no need to use absolute path to executable, as PATH environment
variable provide one and it is easy to be overriden by the user.

/reviewed-on nexedi/slapos.package!31

In order to being able play with generated image many times avoid
needless writes there by using qemu -snapshot switch.

  Include Debian 7 and 8
  Include ansible playbook to automate the build
  Include Ubuntu 15.04 and Ubuntu 14.04
  Include configuration for CentOS 6.7 and 7.2