qa/cephfs: don't pass cmd args from run_as_user as str

qa/cephfs: don't pass cmd args from run_as_user as str

Passing command arguments from run_as_user() to run_shell() as string
can be problematic since command argument to be passed to -c option of
sudo should be a single argument (i.e. 'ls dir' instead of
['ls', 'dir']).

Fixes: https://tracker.ceph.com/issues/46057
Signed-off-by: Rishabh Dave <ridave@redhat.com>