grafana: Fixes to foldersFromFilesStructure option

[root-expert]

This PR includes some additional fixes for the foldersFromFilesStructure introduced in #326 which was incomplete.

1. It correctly handles creating folders from the file structures under {{ grafana_dashboards_dir }} while
2. Registers the created and present dashboards folders and adds a task to remove the folders that should not be present if grafana_provisioning_synced is enabled
3. Fixes folders permissions to be 0755 and files permissions to be 0644 by using u=rwX,g=rX,o=rX

Given a {{ grafana_dashboards_dir }} directory structure of:

├── dash1.json
├── dash2.json
└── folder
    └── dash3.json

the role will create this exact structure under /var/lib/grafana/dashboards.