fix(sign): refuse to create signatures or verifiers with empty keyring
Merge pull request #366 from jonboulle/empty_keyrings

fix(sign): refuse to create signatures or verifiers with empty keyring
feat(X-ConditionMachineMetadata): Add conditional requirements to the unit file.
Merge pull request #162 from calavera/condition_machine_metadata

Add `X-ConditionMachineMetadata` to the unit file.
fix(fleetctl): update ssh.Execute to use a PTY

This allows `fleetctl ssh` to work as expected with interactive commands
(like `toolbox`, or even just `bash`).
This change also reworks the error handling to ensure the exit code of
commands run over SSH are correctly propagated back to the client.
Finally, the semantics for copying input/output streams to/from sessions
are cleaned up (IMHO).
refactor(ssh): rip out unused sync and DRY session logic