aft_python: fix incorrect ndp use

If we want to clear a flag we have to include '--' before the flag we clear,
otherwise ndp will misparse our command (the getopt call will interpret it as
another option, and reject it).

This caused spurious errors at the end of every atf_python test case such as:

============================== 1 passed in 7.26s ===============================
usage: ndp [-nt] hostname

	       ndp [-nt] -a | -c | -p | -r | -H | -P | -R
	       ndp [-nt] -A wait
	       ndp [-nt] -d hostname
	       ndp [-nt] -f filename
	       ndp [-nt] -i interface [flags...]
	       ndp [-nt] -I [interface|delete]
	       ndp [-nt] -s nodename etheraddr [temp] [proxy]

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