Pull Mirroring: ssh public key is not automatically generated

Summary

The SSH public key is not being automatically generated when SSH public key is selected as the authentication method in the mirroring section of a project's settings.

Steps to reproduce

1. Create a new project that is either public or is on a namespace with a Bronze/Starter subscription or higher.
2. Navigate to the Settings -> Repository -> Mirroring repositories section of that project.
3. Select Pull for mirror direction.
4. Optionally enter an ssh:// repository URL.
5. Select SSH public key for the authentication method.
6. Observe that the key is not generated.

Example Project

tristan/mirroring-test

What is the current bug behavior?

Once SSH public key is selected no key is generated.

What is the expected correct behavior?

We should see an SSH public key that the user can provide to the remote server.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com: 11.11.0-ee

ZD: https://gitlab.zendesk.com/agent/tickets/122080 (Internal)

/cc @jramsay