Upgrades with custom osImage are now tested manually.

We should create a new prow job able to configure a custom osImage in the cluster and execute an upgrade.

In order to do so we need to create a new prow step to generate the custom osImage. The custom osImage should be built inside the nodes that will run this osImage, like that we won't have to worry about the architecture.

1. create cluster
2. create custom MCP with one node
3. create a custom osImage
4. apply the custom osImage to the custom pool
5. execute the upgrade
6. remove the osImage