Don't use root_helper when it's not needed

LinuxBridge agent doesn't need to use root_helper when checking if
a device exists. Root priviledges are required to perform operations
inside a namespace and it's not the case here.

Change-Id: I12e0cde2f4dd3b3f8e7f5ed9e90e2805a4466aaa
Closes-bug: #1305800
don't ignore rules that are already enforced

H302 / H304 are clean in the neutron source tree, so it seems
like they shouldn't be ignored. This requires no code changes to
enable.

If there is a longer standing reason for these ignores, would be
great to document in tox.ini.

Change-Id: Ib0f483e4f8fbd328815775a2a6f58be448172bd9
Merge "Don't use root_helper when it's not needed"
Merge "don't ignore rules that are already enforced"