namespace storage specific secret files to 'priv/storage' folder

namespace storage specific secret files to 'priv/storage' folder

As /etc/pve/priv is already pretty polluted, having a
"<storage-id>.pw" file there smells like it could make problems in
the future.

So let the pbs pw file generator use /etc/pve/priv/storages as base
path.
Other storage should move also to that path in the future, if they
save such secrets anywhere in /etc/pve.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>