Restrict backup-list on instance to tenant

Fixed backup-list on an instance, so that we show all available backups for
the instance iff the user making the request is an admin. For a non-admin
user, we return only the backups in his particular tenant.

Change-Id: I2aff6dca053d8261bb70083bf52dac46806faabe
Closes-bug: 1295325
Merge "Restrict backup-list on instance to tenant"