qa: fix misleading "in cluster log" failures during cluster log scan

qa: fix misleading "in cluster log" failures during cluster log scan

Summary:

Fix misleading failure reasons reported as `"… in cluster log"` when
no such log entry actually exists.

The cephadm task currently treats `grep` errors from the cluster log
scan as if they were actual log matches. This can produce bogus
failure summaries when `ceph.log` is missing, especially after early
failures such as image pull or bootstrap problems.

Problem:

first_in_ceph_log() currently:

- returns stdout if a match is found
- otherwise returns stderr

The caller then treats any non-None value as a real cluster log hit and formats it as:

"<value>" in cluster log

That means an error like:

  grep: /var/log/ceph/<fsid>/ceph.log: No such file or directory

can be misreported as if it came from the cluster log.

This change makes cluster log scanning robust and accurate by:

- checking whether /var/log/ceph/<fsid>/ceph.log exists before scanning
- using check_status=False for the grep pipeline
- treating only stdout as a real log match
- treating stderr as a scan error instead of log content
- avoiding overwrite of a more accurate pre-existing failure_reason
- reporting scan failures separately as cluster log scan failed

Fixes: https://tracker.ceph.com/issues/76051
Signed-off-by: Redouane Kachach <rkachach@ibm.com>