Change the way 'debops' script looks for playbooks

'debops' script interprets the first argument given on the command line
and tries to find a corresponding playbook, in order:

- check if given parameter is an existing file and use that as
  a playbook;
- check if given parameter is a <parameter>.yml file in
  'project/playbooks/' or 'project/ansible/playbooks/' directories and use
  that as a playbook;
- check if given parameter is a <parameter>.yml file debops-playbooks
  install directory and use that as a playbook.

If nothing has been found, 'debops' script will default to 'site.yml'
playbook in 'playbooks/', 'ansible/playbooks/' and finally
debops-playbooks install directory.
Allow debops-defaults to run outside project dir
Add support for custom 'ansible.cfg' config file

'debops' script will now generate Ansible configuration file in main
project directory with a set of basic settings, mainly paths where roles
are searched for, various plugins and current inventory path. Additional
options can be added to this file automatically by 'ansible_config_hook'
function defined in '.debops.cfg' configuration file, which is sourced
by 'debops' script.

Default DebOps roles will be cloned into '../roles/' directory relative
to 'debops-playbooks/playbooks' installation directory, to allow for
user roles to be used instead of upstream roles.
Update CHANGELOG.md
Merge pull request #53 from drybjed/revamp-roles

Modifications to Ansible role, playbooks and project directory support