Make restore tool easier to use; Fix docs.

The mongo_restore_database tool, with no backup files listed, would silently
not recover anything.

This changes it to search for bson files under the backup_dir, and recover those.

system.users and system.indexes are skipped, as these have different recovery
requirements, and are better re-added via Salt.

In the process, fix the documentation to reflect the above, and fix the name of the
script!
pymongo is needed for main mongodb sensu checks

The main community mongodb check for sensu is written in python,
and as such needs pymongo
Update CHANGELOG
Merge pull request #2 from ministryofjustice/restore_tool_and_doc_fix

Make restore tool easier to use; Fix docs.