Remove root_helper config from plugin ini

As root_helper is defined in neutron.conf, root_helper in plugin ini
is unnecessary and brings confusion when configuring the parameter.
This patch updates plugin ini of NEC plugin and Brocade plugin.

Change-Id: I5b1c36d8d6ffc20ae0b4191a73aaa51f2b3d9a1b
Closes-Bug: #1259242
Merge "Remove root_helper config from plugin ini"