Switch default fleetctl driver to API

[bcwaldon]

Fix #921

[bcwaldon]

I don't know what I was thinking before, but you definitely cannot hit the fleet API bound on a unix socket through an SSH tunnel. This means, that a command like this will no longer work: fleetctl --tunnel=127.0.0.1:2222 list-machines. The fleet API can be bound on a TCP socket, but this requires extra work an a non-standard configuration. What can we do about this?

/cc @philips @jonboulle

[jonboulle]

Hrm. Could we somehow provide socket activation on localhost by default and then tunnel through to that?

[bcwaldon]

@jonboulle This PR is now rebased on #1035 so we can land the commits that don't actually change the default behavior independently.

We could totally bind fleet.socket on 127.0.0.1:, it just means we will have fleet exposed as a network service, and we'll have to pick the value for that port, two things that we have been trying to avoid.

[bcwaldon]

Updated to preserve behavior of --experimental-api=false on the off-chance that users tried to protect any scripts from this change.

[bcwaldon]

Waiting on this until #1039 makes its way out in a release all the way to the CoreOS stable channel

[cnelson]

FYI, OpenSSH 6.7 contains support for forwarding unix domain sockets. There's an open request to have it updated from upstream:

coreos/portage-stable#173

[crawford]

LGTM