Add FreeBSD and OpenBSD to release builds

[geoffgarside]

It looks like all that would be required is adding "freebsd openbsd" to the end of the OS_PLATFORM_ARG list in cross binary script. Based on loose testing this seems fine, I tried NetBSD as well, but it gets a compile error from one of the dependencies.

[emilevauge]

@geoffgarside can you give us more details on the error you're getting?

[geoffgarside]

Sure

---> Making bundle: crossbinary (in .)
Building binary for linux/386...
Building binary for linux/amd64...
Building binary for darwin/386...
Building binary for darwin/amd64...
Building binary for windows/386...
Building binary for windows/amd64...
Building binary for freebsd/386...
Building binary for freebsd/amd64...
Building binary for openbsd/386...
Building binary for openbsd/amd64...
Building binary for netbsd/386...
# github.com/containous/traefik/vendor/github.com/docker/engine-api/client
vendor/github.com/docker/engine-api/client/client.go:63: undefined: DefaultDockerHost
make: *** [crossbinary] Error 2

Looks like its just "netbsd" missing from https://github.com/docker/engine-api/blob/master/client/client_unix.go#L1