Ensure neutron rootwrap.d symlink is not nested

Before this change, if /etc/neutron/rootwrap.d existed already and
was a dir or a link to a dir, an additional symlink, named rootwrap.d
was created within the rootwrap.d dir, eventually causing the
neutron-rootwrap command to fail and exit with non-zero.

Change-Id: I2f11d7e93ef1445afd1a58e4dd110f7369ec93ab