Instead of excluding only the contents of the datadir, like thus:
```
<path-to-mariadb-instance>/srv/mariadb/**
```

this commit now excludes the data directory itself, like thus:
```
<path-to-mariadb-instance>/srv/mariadb
```

This is important because the mariadb server service script will only
initialise the datadir if it doesn't exist, not if it is merely empty.
Therefore without this mariadb crashes in clone instances.