Cassandra backup/restore

Reasons:
- backup/restore is a significant
features for database management;

Changes:
- Adding backup/restore implementations;
- updating discovery mechanism;
- adding oslo-groups to distinct
backup implementations, to keep code
clean and improve readabilitily.

Change-Id: I7f9d44fdaa7a80841b9325ffab904d5ace39d797
Implements: blueprint cassandra-backup-restore