Add Cassandra DB as one of service types

Trove should support multiple extensions. One of the is NoSQL support.
Main goal is to add support for Apache Cassandra NoSQL database (wiki.apache.org/cassandra).

OS: only ubuntu

Problems:
1. Trove doesn't support multi-service-type reference (https://bugs.launchpad.net/trove/+bug/1215183,
https://blueprints.launchpad.net/trove/+spec/multi-service-type-support)
2. Cassandra doesn't have any ACL inside, so there should be made alot of stubs for Trove API. According to Cassandra specs: use of ACL on keyspaces/columnfailys is not necessary.
3. Cassandra is cluster-orientied database, but trove currently supports single-instance provisioning.

Advantages:
1. 100% replication and cluster availability while node crush.
2. Simple cluster configuring.
3. Simple import/export backups
4. pycassa - very good python client for cluster configuring

Disadvanteges:
1. Cli-based backuping (http://wiki.apache.org/cassandra/Operations#Backing_up_data).
2. Cli-based restoring (same link).