Fix CommandTimeout handling

The refactor broke CommandTimeout handling, which will result
in a NameError. This fixes it by adding a new CommandTimeout
exception in scope and removing the old classes.

Here's an example error message:
```
       
       NameError
       
       ---------
       uninitialized constant Helpers::Docker::CommandTimeout
       
       
       Cookbook Trace:
       ---------------
       /tmp/kitchen/cookbooks/docker/libraries/helpers.rb:104:in `rescue in execute_cmd'
       /tmp/kitchen/cookbooks/docker/libraries/helpers.rb:101:in `execute_cmd'
       /tmp/kitchen/cookbooks/docker/libraries/helpers.rb:117:in `execute_cmd!'
       /tmp/kitchen/cookbooks/docker/libraries/helpers.rb:111:in `docker_cmd!'
       /tmp/kitchen/cookbooks/docker/providers/image.rb:202:in `pull'
       /tmp/kitchen/cookbooks/docker/providers/image.rb:70:in `block in class_from_file'
       
```
if you have a timeout.