Add host management with VirtualBox driver

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Remove spacing in hack/vendor.sh

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Add Digital Ocean driver for hosts

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Add Azure driver for hosts
Only create HTTP transport when required

The current Docker client assumes that there is always a URL
available, but this might not be the case if the host is stopped.
Add maintainers file to hosts

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Add stubs for CLI documentation