libs: add ceph_osd_devices fact listing ceph OSD block devices (#504)

A forthcoming profile needs to set the I/O scheduler to noop/none on the disks backing ceph OSDs, so Puppet must know which block devices those are. This fact surfaces them by inspecting LVM PVs whose volume group belongs to ceph.

- add ceph_osd_devices fact returning the sorted array of PV device paths whose VG name starts with ceph-
- confine the fact to physical Linux hosts with pvs installed so it never resolves on VMs

https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT
Reviewed-on: #504
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

modules/libs/lib/facter/ceph_osd_devices.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require 'facter'
+
+Facter.add(:ceph_osd_devices) do
+  confine kernel: 'Linux'
+  confine { Facter.value(:is_virtual) == false }
+  confine { Facter::Core::Execution.which('pvs') }
+  setcode do
+    devices = []
+    output = Facter::Core::Execution.execute(
+      'pvs --noheadings -o pv_name,vg_name 2>/dev/null',
+      on_fail: nil
+    )
+    output&.each_line do |line|
+      pv_name, vg_name = line.split
+      next unless pv_name && vg_name
+
+      devices << pv_name if vg_name.start_with?('ceph-')
+    end
+    devices.sort
+  end
+end