Remove duplicate logouts with private registry with unit tests.

[jrpalma]

- What I did
Fixed the problem of docker logout trying to erase credentials multiple times, which leads to an warning when credentials no longer exist. This happened with docker-credentials-pass on Linux on Ubuntu 18.04. Fixes #204

- How I did it
I did not add the registry to be loged out if it is a private registry. Hence, removing the duplicate entry.

- How to verify it
I created unit test to ensure the credential is not erased twice.

- Description for the changelog
Do not erase private registry credential multiple times.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master-duplicate-cred-erase" [email protected]:jrpalma/cli.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.