Fix inconsistent usage of mount_point

Reasons:
 - multiple datastores;
 - inconsistent usage of mount_point at
   taskmanager and guestagent;

Changes:
 - oslo groups;
 - mount point per datastore;
 - guest API not changed.

Change-Id: I8044789c6ab2ed837ff0e1b16183dfeafbf955bb
Closes-Bug: #1248212
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