Add initial version of script that synchronises missing reports

The script gets all yaml files from a remote server via rsync, checks
if the yaml reports are present in the raw report or santised directories
and copies them if they are not there

Also adding an example remote_servers file that contains the list of
remote servers we want to check
We also need to add an extra check here to be sure that the report is not inside of the database.

You can do this by querying mongodb like so:

```
client = MongoClient(db_host, db_port)
db = client.ooni
db.reports.find_one({"probe_asn": probe_asn, "start_time": start_time, "test_name": test_name, "input_hashes": input_hashes}):
# Skip this one too
```