prestart & poststop hook: init REST environment, e.g. for storage activation

prestart & poststop hook: init REST environment, e.g. for storage activation

Initialize the basic CLI REST environment which is expected on some
PVE methods we may rely on.

This became a specific problem recently when adding better support
for external and/or multiple ceph RBD clusters on a PVE system in
commit cfe46e2d4a97a83f1bbe6ad656e6416399309ba2 from pve-storage,
which added a PVE::Rados call to get the underlying cluster FSID
required to build the /dev-mapped RBD path, and PVE::Rados
requires a initialized RPC/REST environment.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Suggested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>