Update all LWRPs to check that docker is alive.

Before issuing any commands that talk to the local docker daemon,
make sure it is running (since its service starts in the background).
Move docker_cmd and execute_cmd methods to helper

Move docker_cmd and execute_cmd into the helper mixin to remove
some repeated code.
Add docker_cmd! to raise exceptions on failure.

Update all call sites to use the failing version of docker_cmd.
Fix removal of tagged image.

`docker rmi` defaults to `latest` if no tag is specified. But if you
have a tagged image, there might not be a latest, in which case you
must qualify the image with a tag name.
Merge pull request #62 from jcrobak/return-code-validation2

Return code validation