Add CentOS Atomic 7.3 Image

[eedri]

We are planning to add a new suite which will run Kubenetes on Lago VMs.
Make sense to install Atomic rather than normal CentOS on those VMs to optimize containers support.

[ifireball]

We will probably need cloud-init support in Lago to properly use Atomic images.

[eedri]

@gbenhaim can we include this work as part of the next Lago release?

[gbenhaim]

libguestfs tools cannot be applied on atomic images (because of OSTree [1]), so
as @ifireball we will need to support cloud init in lago.
I've already started to work on it and will try to push it to the upcoming release.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1102241