Fix IpNetnsCommand to execute without root_wrapper when no netns

IpNetnsCommand accept execution with a network namespace, and
that requires root privileges and a root_wrapper.
IpNetnsCommand does accept no namespace too, in that case, the
root_wrapper doesn't have to be present necessarily, unless the
command we're executing requires root privileges itself.

This patch fixes the check condition on IpNetnsCommand execute
method.

Change-Id: I5ab2f3f1daf4a5a080611dbcd3dbd43292f6766a
Closes-Bug: #1358196
Merge "Fix IpNetnsCommand to execute without root_wrapper when no netns"