Set haproxy nbthread[1] to 2

The haproxy that gets spawned by agents can cause `too many open files`
exceptions on very large deployments. Since haproxy is not resource
intensive process, we can limit it to only use 2 threads to mitigate
this issue.

[1] www.haproxy.com/documentation/haproxy-configuration-manual/latest/#nbthread
Closes-bug: #2120668
Change-Id: I70b2461d65f3e9967bea1239eac3c1ccb52b3138
Signed-off-by: Miro Tomaska <mtomaska@redhat.com>

neutron/common/metadata.py

@@ -45,6 +45,7 @@ global
     user        %(user)s
     group       %(group)s
     maxconn     1024
+    nbthread    2
     pidfile     %(pidfile)s
     daemon
 

releasenotes/notes/haproxy-nbthread-a658040c98639b47.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Limit the number of threads used by the metadata haproxy service to 2 using
+    the nbthread configuration. This prevents haproxy from spreading across many
+    threads when the process lacks other restrictions like CPU affinity.
+    For more information, see bug
+    `2120668 <https://bugs.launchpad.net/neutron/+bug/2120668>`_.