Feat: Add ability to set custom image registry

[ClusterJan]

Add a global variable to override the image registry to allow the usage of a private registry.

As my changes include a new registry option for the database image, this seems to be a breaking change. But I don't know if this might be obsolete, because of the issue #70.

[jhbeskow]

@vgrassia, there is a separate PR (#89) that also does something similar here. I will leave it to your discretion on which you prefer. That one is a bit more granular on the registries as you would provide them as part of each image's name.

[vgrassia]

Thank you for bringing this up. We've added the ability to customize each individual image registry with PR #89.