Support Titan DB

In Vitrage we plan to use Titan DB (http://thinkaurelius.github.io/titan/) in order to store the Entity Graph.
The Entity Graph (https://blueprints.launchpad.net/vitrage/+spec/entity-graph) is a graphical representation of the cloud entities and alerts, and the relationship between them. As such, the graph we need to maintain is a dynamic graph, which is frequently updated to reflect the state of the cloud.
Furthermore, we need to store Action Templates (https://blueprints.launchpad.net/vitrage/+spec/templates), which might also be represented in graph structure.
Due to the the potentially large size of a cloud, we require a scalable graph database that can support fast queries and storage of new data. For this, we selectes Titan DB as the (first of possibly several) graph database backend to store all this information.