Adapt 'Webviz deploy radix' to support non-public repos

[antoneskov]

Ref #592 - see the issue for problem description

Suggestion
Adapt the workflow for non-public repos in such way that it is automated as such as possible:

• Investigate whether it is possible to generate ssh key pair automatically; if it is not - give instructions to do it manually before the process starts;
• Detect whether the repo to be deployed is not public, either automatically, or by specifying a command-line flag;
• For such repos:
  • Add "build secrets" section to radixconfig.yaml
  • Set up public deploy key in Github
  • Set up private key forRadix app in Build secrets (one initial build will fail, but the secrets will be generated)