set proper mongo data directory

[malaskowski]

Fixes #42
By mistake, AET MongoDB data folder was set to the default value for the sc-mongodb cookbook which is /var/lib/mongo/ directory.
After this fix, MongoDB will be configured to store its data under /opt/aet/mongodb/db.

This is critical bugfix for the instances with separate disk/lvm configured for MognoDB.

Important!
If your instance is provisioned with aet-cookbook versions v5.0.0 or v5.1.0 you need to manually move your MongoDB data. Please follow steps below after upgrading your instance with version of cookbook that contains this fix:

1. Stop Karaf instance with sudo service karaf stop
2. Stop MongoDB instance with sudo service mongod stop
3. Clear /opt/aet/mongodb/db with sudo rm -rf /opt/aet/mongodb/db/*
4. Move all the data from the old director to the new with: sudo mv -f /var/lib/mongo/* /opt/aet/mongodb/db (depending of the size of your database it may tak a while, if you wish you may use rsync instead of mv).
5. Start MongoDB instance with sudo service mongod start. Check if all the databases are visible after data migration.
6. Start Karaf instance with sudo service karaf start