Switch nightlies to head/2.15

[thehejik]

We will keep using main branch for building dev turtles until we start work on 2.16, then we will switch 2.15 to use release/v0.27 branch here.

Problem is that even though the values.yaml from our custom built turtles chart is referencing the right image localhost:5000/rancher/turtles it is still overwritten to rancher/turtles by system-chart values.yaml - so far the reason is unknown.

Edit: Problem is here, from original values.yaml another modified values.yaml is generated and replaced by image.repository: rancher/turtles, eventually with systemDefaultRegistry/ in front of it. This is known on head/2.13 and Prime (but not on head/2.14).