ceph: Add default value for MK_CONFDIR

When the environment variable `$MK_CONFDIR` was not set, the ceph
agent would crash with a KeyError.

This has been fixed and the default directory is now
`/etc/check_mk/`, consistent with other agents.

CMK-32386
#894

Change-Id: I5f92426b17da95ec51125c8393334f7564af74cb

Author: fdriessler

.werks/19869.md

@@ -0,0 +1,18 @@
+[//]: # (werk v3)
+# ceph: Fix agent crash when MK_CONFDIR is not set
+
+key        | value
+---------- | ---
+date       | 2026-06-19T01:53:09.527671+00:00
+version    | 3.0.0b1
+class      | fix
+edition    | community
+component  | checks
+level      | 1
+compatible | yes
+
+When the environment variable `$MK_CONFDIR` was not set, the ceph agent would
+crash with a KeyError.
+
+This has been fixed and the default directory is now `/etc/check_mk/`,
+consistent with other agents.

cmk/plugins/ceph/agents/mk_ceph.py

@@ -141,7 +141,7 @@ def main() -> int:
     except ImportError:
         return _bail_out_missing_dependency()
 
-    ceph_config, ceph_client = _load_plugin_config(os.environ["MK_CONFDIR"])
+    ceph_config, ceph_client = _load_plugin_config(os.getenv("MK_CONFDIR", "/etc/check_mk"))
 
     cluster = RadosCMD(Rados(conffile=ceph_config, name=ceph_client))
     cluster.client.connect()