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
Default RABBIT_HOST to be the SERVICE_HOST.

Using localhost for RABBIT_HOST can cause problems - particularly
on systems where IPv6 is partially broken and localhost resolves
to an IPv6 address.

The service should logically be running on SERVICE_HOST unless
otherwise overridden.

Change-Id: I6efb71b8b95174770066f38403d5d428ec88c3b6
Merge "Fix Neutron plugin XTRACE handling"
Merge commit 'refs/changes/49/81849/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/75/84475/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch