Limit k3d cluster name length

[smlx]

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for changelog and subsystem label(s) applied

This PR limits the length of the k3d cluster name to 35 characters to avoid error messages like this:

time="2020-04-30T14:57:46+02:00" level=fatal msg="Cluster name is too long (37 > 35)"
Makefile:1009: recipe for target 'k3d' failed
make: *** [k3d] Error 1

Characters are removed from the front of the CI_BUILD_TAG to get down to 35 chars so that the incrementing CI build number at the end of CI_BUILD_TAG is retained in the cluster name. This way the name remains unique.

I also removed an unused Makefile variable.

Closing issues

n/a