Add playbooks to specify multiple playbooks to run

[syndbg]

Adds playbooks config.

Why

When running "complex" playbook provisioning scenarios before testing, you may need to re-use different playbooks.

For example I was testing playbooks.

However for the release process provisioning from scratch you execute e.g playbook db.yml and web.yml.

They're separate and if I was to create a new playbook that includes both roles used, I wouldn't test the actual playbooks executed.

So with this change usage is

provisioner:
  name: ansible_push
  playbooks:
    - db.yml
    - web.yml

Anything other than an Array will be marked as invalid configuration. E.g

provisioner:
  name: ansible_push
  playbooks: {}

Important

Includes #51

@ahelal