We cannot restore a backup to a larger volume on ceph because it
fails with status "error_restoring". This patch adds read/write
permissions to volumes pool for backup user. We need such permissions
to change volume size during restoring backup when the backup is
smaller than a volume.
Change-Id: I794c1126bcee4e07baf5a9dcfee779fd61da5636
Closes-Bug: 1519749