Fix incorrect loading of container name when links are present.

[jperville]

Bug summary

The docker_container provider will incorrectly attempt to recreate the docker container if the container already exists and has other containers linking to it.

How to reproduce

1. declare a docker_container resource (with docker run --name='my-db')
2. declare another docker_container resource that link to the first one (eg. docker run --name my-app --link my-db:db)
3. run chef-client, it passes (and the link can be seen in output of docker ps)
4. run chef-client again, docker will fail with the following message:

Error response from daemon: Conflict, The name my-db is already assigned to d0ac4cf80ab1. You have to delete (or rename) that container to be able to assign my-db to a container again.

Here are my containers as listed in docker ps:

a6ebcb14df56        registry.local/my-db:latest   "/sbin/my_init --ena   44 minutes ago      Up 44 minutes       1337/tcp     my-db                   
d0ac4cf80ab1        registry.local/my-app:latest   "/sbin/my_init --ena   45 minutes ago      Up 45 minutes       80/tcp     my-app,my-db/db   

The reason this happens is because the docker_container provider uses the raw name (as listed in docker ps) as default value for @current_resource.container_name; in my example, this means that the providers loads my-app,my-db/db as the container_name instead of my-db ; since the provider (incorrectly) filled the container_name attribute then it will wrongly think that the container does not exist and attempt to create it with the run action ; then docker run will fail with the above message because the container named my-db already exists.

Suggested fix

This simple PR should fix the problem by trying to guess which is the main container name from raw docker ps output. I considered that every names which contain a '/' character are link names and should be ignored, this works fine enough for me.

[jperville]

After further investigation, the problem is not specific to links, it just happens to show up when links are used. The actual problem is that the container_matches? method in the container provider is to lax and will accidentally match the wrong container, thus breaking my links further in my test suite.

I close this PR and will open a new one shortly (referencing this PR).