Add option to use private images

[LilliJane]

This pull request will close #378.

• Add --private option that will ask the user for their username and their password.
• Label the secrets with registry, project and image names.
• Extract image registry from the provided image name (e.g quay.io/repo/image to quay.io, repo/image to index.docker.io).
• Use environment variables as username and password for the secret
• List secrets before creating a new one to search for a secret matching with the current image
• Tests secret creation
• Tests secret listing
• index.docker.io
• quay.io
• --clean-creds flag
• gcr.io
• aws
• azurecr
• other registries (?)

Edge case:
If users make a typo in their username/password and the cli automatically reuse a secret, users won't be able to modify their secret which will lead jobs to fail. This is why people can use the --clean-creds flag with the nerd job run command to be asked again for their credentials.