Add support for storage backends for SSH keys.

The SSH client functionality is now separated from SSH key storage.
SSHClient has no knowledge now of where keys live. It could be on the
file system, in memory, sitting on a gopher server, or whatever else the
storage backend is talking to.

All the file access has been moved to a new FileSSHStorage class.

New unit tests have been added to test the new code paths.

Unit tests all pass.

Tested my local deployment with the SSH-backed Review Board Git repo.
Everything worked fine.

Tested running rbssh manually to connect to servers.