always downcase the container name

[robuye]

Docker fails when it attempts to create a container with capital letters in name. Stack trace:

>>>>>> ------Exception-------
>>>>>> Class: Kitchen::ActionFailed
>>>>>> Message: 1 actions failed.
>>>>>>     Failed to complete #create action: [no such image: 0d9c95481b-LP--ruby-ubuntu-1604: invalid reference format: repository name must be lowercase
] on LP--ruby-ubuntu-1604
>>>>>> ----------------------
>>>>>> Please see .kitchen/logs/kitchen.log for more details
>>>>>> Also try running `kitchen diagnose --all` for configuration

D      ------Exception-------
D      Class: Kitchen::ActionFailed
D      Message: 1 actions failed.
>>>>>>     Failed to complete #create action: [no such image: 0d9c95481b-LP--ruby-ubuntu-1604: invalid reference format: repository name must be lowercase
] on LP--ruby-ubuntu-1604
D      ----------------------
D      ------Backtrace-------
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/lib/kitchen/command.rb:183:in `report_errors'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/lib/kitchen/command.rb:174:in `run_action'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/lib/kitchen/command/action.rb:36:in `block in call'
D      /usr/lib/ruby/2.4.0/benchmark.rb:293:in `measure'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/lib/kitchen/command/action.rb:34:in `call'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/lib/kitchen/cli.rb:52:in `perform'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/lib/kitchen/cli.rb:193:in `block (2 levels) in <class:CLI>'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/bin/kitchen:13:in `block in <top (required)>'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/lib/kitchen/errors.rb:171:in `with_friendly_errors'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/gems/test-kitchen-1.20.0/bin/kitchen:13:in `<top (required)>'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/bin/kitchen:23:in `load'
D      /home/rob/projects/chef-repo/vendor/ruby/2.4.0/bin/kitchen:23:in `<main>'

The container name was infered from the suite name, it's LP__ruby in my case.

Dokken supports an option to provide custom name, but it shouldn't be necessary, I think the default name should always work.

This will downcase the container name, everything else in the output will remain as it was (logs still output capital letters for the suite name etc).

It fixes the problem I had, but I can't run the tests locally becuase my docker setup has user namespaces enabled and it requires changes from the P/R #148 to be pulled in. That said I have done the best I can to ensure the changes added here work as expected.