qa/cephfs: don't strip any whitespace for get_shell_stdout

qa/cephfs: don't strip any whitespace for get_shell_stdout

Whitespace is not removed from the end of the stdout returned by the
method get_ceph_cmd_stdout(). Follow the same policy here since it is
better to not do so (this whitespace can be useful, when copying Ceph
auth keyrings from stdout to a file) and also for sake of uniformity of
interfaces.

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit 9582b9b13a42fae4c7f38c22f9202eb893b6f1bc)

Conflicts:
qa/tasks/cephfs/mount.py
- Conflict occured for 2 reasons -
  - One, method get_shell_stdout() is absent on Reef branch but not in
    main so this patch which makes modification to it will obviously run
    in to conflict
   - Two, run_shell_payload() lies right next to get_shell_stdout() in
     main branch and its definition is quite different, leading to
     conflict again.