Add service : Etherpad

Include this role : https://github.com/systemli/ansible-role-etherpad

• Check if role can support plugins, if not, add functionality
• Add role
• Add variables
• Deploy role on testing infra
• Deploy role to prod infra
• Import existing data to prod infra
• Add backups ( linked to #11 (closed) )
• Connect to the deleted pad function ( https://git.laquadrature.net/la-quadrature-du-net/outils/etherpad-cleanup )