Introduce basic support for provisioning nodes with centos7

This change introduces support for provisioning nodes with centos7
using bifrost. Please note that the node where the engine is run on
must still have ubuntu1804 since python3 support on centos7 is not
fully available. Lack of certain python3 packages such as selinux
causes issues for Ansible template module, making it nearly impossible
to use Ansible on centos7 in python3 setup.

It is also important to note that the package built for centos7 probably
has unnecessary distro dependencies coming from ubuntu1804. These
dependencies does not cause any issues for provisioning and they only
waste space. They will be adjusted once the dependencies are distributed
across stacks.

Depends-On: I58965e979a2be099a0c88c328cc87dafd88d3c16
Change-Id: Iba181f25851feb239a7ba1c0eb38569726c40fa3
3 files changed