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