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.

[voidquark]

I can also see that this could cause issues, as __dashboards_copied is being overwritten between two tasks.
In my opinion, this needs to be properly tested for edge cases, since users can provision dashboards both via Grafana.net and from a folder on the deployer machine.

[ishanjainn]

@root-expert Could you resolve conflicts, Should be good to merge post that right @voidquark ?