Add clearer help kwarg for max_volumes_per_user

Add same wording as comment in trove.conf.sample.

Change-Id: I0ad8ca2dd2d6aaa4b7012c491c4a467b6004e4cb
Closes-Bug: #1274608
Raises BadRequest for grant_access on ignore_dbs

Reasons:
- Currently, it just logs an informative log in guestagent log, and then
  control shifts to next line, which provides the requested grant.
- This brings the need to raise the exception instead of informative
  logging.

Changes:
- Raises BadRequest if ignore_dbs are requested access instead of
  informative logging into guest-agent logs.

Change-Id: I9df08d04cd42916f3d1f91e05cd4799bb9041eae
Closes-Bug: #1257021
Merge "Raises BadRequest for grant_access on ignore_dbs"
Merge "Add clearer help kwarg for max_volumes_per_user"
Make backup/restore pluggable

Reason:
 - mutliple datastore support;
 - execute_backup expects that instance that
   is going to be backuped is a mysql server
   (expects that my.cnf exist).
Changes:
 - oslo groups added;
 - updating MySQLDump strategy;
 - cleaning runner parameters;
 - updating tests;

Change-Id: Id98ac37c048db79cd24adfa22f0db722648cf55b
Closes-Bug: #1281127
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;

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