Changed logtype to file instead of owncloud.

- Updated the config sample to point to log_type='file'
- Renamed the Class for logfile logging to File in namespace 'OC\Log\'.
  Changed the occurrences of 'OC\Log\Owncloud' to 'OC\Log\File'.
- Renamed the Class for log:file command to File in namespace 'OC\Core\Command\Log\File'.
  Changed registration of the command to use 'OC\Core\Command\Log\File'.
- Changed default Syslog tag to Nextcloud
- Retained backwards compatibility for configs with 'logtype' => 'owncloud'

- Adjusted tests for the new file log.

Closes #490.