Output file prefix added as a parameter

Output file prefix is now configurable. By default it still uses `gethostname` function but now you can actually override it by setting `output_file_prefix` (It may come handy for those hosting multiple symfony2 applications that use this bundle)

Example configuration:
```
dizda_cloud_backup:
    output_file_prefix: site1
    cloud_storages:
        dropbox:
            user: user@example.com
            password: password
```