Added step to make sure 'root_helper' key references 'neutron'

In the current Neutron Havana packages for RHEL-style distributions,
the default 'root_helper' key in neutron.conf references 'quantum'
instead of 'neutron'. This issue should get fixed upstream, but in
the meantime I added a step to make sure this key references
'neutron'.

Change-Id: I25f9faaba1067040829633aff990d238f5bf4ced
backport: havana
Closes-Bug: #1258309
(cherry picked from commit 5dc7f610d57b79ac133b48725df1c669f525add8)
Merge "Added step to make sure 'root_helper' key references 'neutron'" into stable/havana