Allow SSH keys to be backed to KeyVault

[alfpark]

Provide a new option to allow ssh private keys to be stored in KeyVault and referenced in config files.

• [ ] ssh_private_key_keyvault_secret_id in all ssh blocks
• [ ] Automatically convert and populate ssh_public_key_data
• [ ] auto_keyvault option will automatically/transparently generate/store/retrieve/gc ssh keys per pool/remotefs cluster