Change base image to SLE Micro for Rancher

[mudler]

As SLE Micro for Rancher is out (http://registry.suse.com/suse/sle-micro-rancher/5.2) we can now base os2 on that image.

This card is potentially tight to rancher/elemental-toolkit#1238 as changing directly the base image might not just work due to layout incompatibilities with leap.

• Dockerfile https://github.com/rancher-sandbox/os2/blob/master/Dockerfile#L1

[mudler]

Blocked by rancher/elemental-toolkit#1238

[mudler]

As part of this card I'll change a bit the build process to take into account of several aspects:

• sandboxed builds (Ideally the same build process should be working on OBS/IBS)
• split os2 and ros-installer