qa/cephfs: override testing kernel with -k option

qa/cephfs: override testing kernel with -k option

Normally, the fs suite overrides the kernel branch whenever the kernel client
is used, according to the matrix of configs. This prevents easily testing a new
kernel with the -k option to teuthology-suite. So, using the base config passed
to the script, detect if an alternate testing kernel is desired and use that
instead.

The scheduler still needs to do some work:

    $ teuthology-suite ... --verbose -k wip-pdonnell-i66704 --filter k-testing
    ...
    2024-10-18 00:29:43,875.875 DEBUG:teuthology.suite.merge:base kernel {'branch': 'wip-pdonnell-i66704', 'kdb': 1, 'sha1': '745cacd8f31e50d7f3b6039bbd8c9a8dfc07bf03', 'flavor': 'default'}
    2024-10-18 00:29:43,875.875 DEBUG:teuthology.suite.merge:overriding testing kernel with {'branch': 'wip-pdonnell-i66704', 'kdb': 1, 'sha1': '745cacd8f31e50d7f3b6039bbd8c9a8dfc07bf03', 'flavor': 'default'}

vs.

    $ teuthology-suite ... --verbose --filter k-testing
    ...
    2024-10-17 20:04:52,265.265 DEBUG:teuthology.suite.merge:base kernel {'branch': 'distro', 'kdb': 1, 'sha1': 'distro'}

Which will only select jobs using the "testing" kernel.

See-also: https://github.com/ceph/teuthology/pull/2008
Fixes: https://tracker.ceph.com/issues/68603
Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>