Create fresh SSH private key for SSH storage

At the moment, the SshStorage only works in single-user-mode when the user running repomaker has a private ssh key available and added it to the remote host.

In preparation for multi-user-mode, but also for single-user-mode, we want to be able to generate a new fresh ssh-key and store it under PRIVATE_REPO_ROOT. This key will then be used to make ssh connections (via fdroidserver and rsync) to the remote host.

The public key should be displayed to the user along with instructions for adding this key to the remote server where the repository will be published.