Fix parsing and storage of generated SSH keys

Summary: Fixes T4772. We weren't parsing generated public keys properly, and were storing them in the wrong format.

Test Plan:
  - Updated a private key.
  - Generated a public key.
  - Saved the public key.
  - Used a generated private key to authenticate.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4772

Differential Revision: https://secure.phabricator.com/D8721