Fix https://github.com/docker/machine/issues/1651
As pointed out in the issue above, the `active` and `ls` commands used
different methods for determing the active machine. This commit defines
a single method on the `host` struct called `IsActive` which provides
a uniform check for machine activness. `IsActive` returns true only
if `DOCKER_HOST == url` and the state is not stopped - previously the
`active` command only checked the url.
* Add a single `host` method `IsActive` for determining if a machine is
active.
Signed-off-by: Matt McNaughton <mattjmcnaughton@gmail.com>
003770f2d4