Add tasks for generating TLS certificates

[p-j-smith]

Add the remaining tasks from our existing docker role. These tasks create CA, server, and client certificates.

• add a variable docker_generate_certificates . If true, certificates will be generated. Defaults to false
• add separate task files for creating CA, server, and client certificates
• a client certificate is created for each client listed in docker_client_hostnames . The certificates are copied to the Ansible controller
• add description of all new variables to the README
• add to the README a short discussion of when you would want to generates the certificates

[drmatthews]

Couple of small typos, otherwise looks good - thanks!