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
Merge "Make backup/restore pluggable"