Added 'secret' role and moved site.yml

- 'secret' role allows you to create encrypted storage that can be used
for secure storage of passwords, certificates and other important files.
It replaces and complements old "secret/" directory functionality. More
information can be found in README file in the role subdirectory;

- 'site.yml' (main playbook) has been moved from root of the project to
'playbooks/site.yml' to keep it in line with other playbooks;