Add support for config groups for Mongo 2.4.9

Blueprint https://blueprints.launchpad.net/trove/+spec/mongodb-configuration-management adds support for config groups for Mongo 2.0.4.

Mongo 2.0.4 consists of only non-dynamic config parameters, i.e. all parameters require a restart.
Mongo 2.4.9 has the added capability of supporting dynamic parameters. http://docs.mongodb.org/v2.4/reference/command/setParameter/#dbcmd.setParameter

Inorder to support dynamic parameters, pymongo could be used to perform the required operation.

Since differences exist in config group implementation of different versions of Mongodb & also to support multiple versions, we could add a manager per version to handle the implementation.

Example :
 'mongodb': 'trove.guestagent.datastore.mongodb.manager.Manager',
 'mongodb24': 'trove.guestagent.datastore.mongodb.manager.Manager',