qa/cephfs: don't use exclam mark in tests in test_cephfs_shell.py

qa/cephfs: don't use exclam mark in tests in test_cephfs_shell.py

Exclamation mark is a special character for bash as well as
cephfs-shell. For bash, it substitutes current command with matching
command from command history and for cephfs-shell it runs the command
as OS-level command and not inside the cephfs-shell.

And evey command executed in tests (say "ls") is run by passing it as a
parameter to cephfs-shell command (that is "cephfs-shell -c <conf> --
ls"). So, exclamation mark, when used in tests, is consumed by bash
instead of cephfs-shell.

To avoid these complications it's best (and even simpler!) to issue the
command meant for bash on bash without going through cephfs-shell.

Fixes: https://tracker.ceph.com/issues/55394
Signed-off-by: Rishabh Dave <ridave@redhat.com>