Graylog2-server: Add Backup / Create Plugin to create Configuration Backup

[jalogisch]

Idea description

Coming from the Discussion in https://github.com/Graylog2/documentation/issues/10 it would be helpful if it is possible to create a Backup of your Graylog Configuration / Settings periodical.

What

Create a Plugin that dumps the mongodb and get the graylog configuration to a central location. Restore should be possible too.

[runningman84]

Maybe you could even support a configuration like this

# backup mode
cfg_backup_mode = hourly
# backup path
cfg_backup_path = /data/graylog.settings
# restore automatically if mongodb is empty
cfg_restore_path = /data/graylog.settings

Right now we backup the whole mongodb database, a single backup file would be easiyer to handle.