Add support for SL Micro "Extras" repository for SL Micro 6.1

[jdob]

In #591, a temporary fix was added that hardcodes the extras URL to be aligned to the 6.0 release. This issue is to track the work towards detecting the SL Micro version and introducing it to the resolution script as a variable.

The current thinking is to extract /etc/os-releases from the base image, read the version from there, and keep it internally somewhere in the context as the image is built so it can be substituted into script template.