Bump all releases to use SLE15 stemcell

[bikramnehra]

Is your feature request related to a problem? Please describe.
Currently most releases seem to have been using SLE_15_SP1 stemcell except external releases e.g:

https://github.com/SUSE/kubecf/blob/master/deploy/helm/kubecf/values.yaml#L59-L73

Describe the solution you'd like
Make sure the CI pipelines for external releases are updated and kubecf is making use of those release images.

[bikramnehra]

Currently there is an inconsistency in kubecf, where some images are being fetched from registry.suse.com and others from DockerHub. Since we want all the images to be fetched from DockerHub would want to modify docker registry used in cf-ci repo.

The final goal is to converge both set of pipelines: #220

Also, cf-mysql release is failing to build for which an issue is already opened:
#270