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.