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