Prevent roles to be played several times

[vincentdoba]

Currently, some roles are played several times, due to their presence in dependencies for other roles and their presence in main site.yml. For instance the role nginx is played as part of site.yml and as dependency of owncloud, blag and roundcube role

We should add triggers at end of such roles to prevent running them several times, as suggested here : https://groups.google.com/forum/#!msg/ansible-project/erqBH5SmPUo/EG01f4X6rhQJ

[vincentdoba]

The _setfact is not satisfactory as it prevents to play role for every following runs. Another solution would be to play certain roles only as dependencies for other roles but it seems to prevent us to run these roles played as dependencies alone, as in this case they are removed from site.yml.