Fix SSH pull mirrors not working

This fixes a regression caused by https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9134, which causes an already-created SSH public/private key to be regenerated. When the Detect Host Keys button is clicked, a Sidekiq worker generates an SSH private and public key, the latter of which is shown to the user. Creating a new mirror would wipe that data and cause a new key to be generated unnecessarily.

The fix adds a hidden element so that the authentication mode will be sent along the password to remove the need to clear the credentials entirely.

Closes gitaly#1550 (closed)