ceph: introduce '/etc/pve/ceph'

ceph: introduce '/etc/pve/ceph'

This commit adds the '/etc/pve/ceph' directory to our overall expected
Ceph configuration.

This directory is meant to store cluster-wide, non-private
configuration files used by Ceph applications and services that are
executed with lower privileges, such as 'ceph-crash.service'.

The existence of the directory is now also checked for when checking
whether Ceph is configured correctly. This makes it easier for our
other tooling to rely on the directory's existence, reducing the
number of otherwise needless frequent checking.

* For new clusters: `pveceph init` now creates '/etc/pve/ceph' when
  called.

* For existing clusters: The 'postinst' hook this commit adds ensures
  that '/etc/pve/ceph' is created when updating.

Signed-off-by: Max Carrara <m.carrara@proxmox.com>
Tested-by: Friedrich Weber <f.weber@proxmox.com>