Enabling docker.user causes whiskhost playbook to fail.

## Environment details:
Distributed Ubuntu.

## Steps to reproduce the issue:

1. enable docker.user setting in group/all
2. ansible-playbook -i environment/distributed prereq_build.yml

It failed due to undefined variable {{ cred_home }} while trying to set authorized file in addUser.yml.

`- name: add {{docker.user}} ssh key to authorized_keys
authorized_key: user={{docker.user}} key="{{ lookup('file', '{{ creds_home }}/ssh/id_rsa_{{docker.user}}.pub') }}"
become: true
`
also I could not find, this public key being generated in any playbook, Is it expected to be created manually before? I think that can be generated once during setup.yml if docker.user enabled or some other playbook and then can be used in addUser.yml.