storage: add Instance type

This will represent the Instance model.
managers/ec2: implement info
managers/ec2: implement is_ok
ec2/managers: change calls to Storage.retrieve
managers/ec2: store the instance object
storage: add method to_dict to Instance
storage: implement MongoDBStorage
requirements: add pymongo
travis: start mongodb
requirements.apt: remove file

We will not depend on sqlite3 anymore.