Support full backup/restore for Cassandra

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

Changes:
 - adding required configurations for resolving
   backup/restore strategy and its namespaces;
 - adding backup/restore implementations;
 - updating discovery mechanism;
 - adding oslo-group attribute to distinct
   backup implementations, to keep code
   clean and improve readabilitily;
 - unit/int tests proposed.

Change-Id: I6da2cf726bdb4ab2b969b745f32429e3b2c98bdb
Implements: blueprint cassandra-backup-restore