Refactor the installer envvars

[mrsimonemms]

As part of #12687, there were a number of suggested changes to the envvars made the in the review. This ticket is to implement these suggestions.

1. expand all REG_ envvars to REGISTRY_
2. change REG_INCLUSTER_STORAGE_S3_BUCKETNAME to REGISTRY_INCLUSTER_STORAGE_S3_BUCKET_NAME
3. any other envvars as you see fit

This will need to change in:

• envvars.go (and it's test)
• gitpod-kots-config.yaml
• kots-install.sh
• check for any other places