qa/cephfs: run nsenter commands with superuser privileges

qa/cephfs: run nsenter commands with superuser privileges

And add a method that sets self.fuse_daemon.subproc.pid to the PID of
the process that doesn't have sudo in its arguments. For example, when
"sudo ceph-fuse /mnt/cephfs" is run on the shell, it launches process
with arguments "ceph-fuse /mnt/cephfs". The added method gets PID of
latter/child process and sets that as the fuse daemon's PID. Not doing
so kills the former/parent process but the not the child process.

Also, since we are around cleanup this method a bit.

Fixes: https://tracker.ceph.com/issues/45339
Signed-off-by: Rishabh Dave <ridave@redhat.com>