Fix Neutron plugin XTRACE handling

The various Neutron plugin files need to have unique variables for the
xtrace state as they are sometimes nested more than two levels deep
and MY_XTRACE is getting stomped.  This gives each of the neutron_plugin
and neutron_thirdparty include files a unique XTRACE state variable.

I don't think this is a problem with any of the other plugin include
files (yet) so this just handles Neutron for now.

Change-Id: I7c272a48e7974edecaff5f431ff7443dd6622588
Merge "Fix Neutron plugin XTRACE handling"
Merge branch 'master' of https://github.com/openstack-dev/devstack