Fix validation of SSH keys with spaces in the comment field

Summary: Fixes T5449. Keys are in the form `<type> <key> <comments>`, where comments are optional and can have spaces.

Test Plan:
Tried these invalid keys:

  - Empty.
  - One part.
  - Invalid type.

Tried these valid keys:

  - No comment.
  - Normal comment.
  - Comment with spaces.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5449

Differential Revision: https://secure.phabricator.com/D9701