Clear all l3 related namespace before starting neutron-l3-agent only when l3_ha is enable

If we are not using l3 ha mode, it’s not necessary to delete the namespaces related to l3. It will speed up the start of the neutron l3 agent.

Change-Id: I78f6d927a78e8f9e4ed855e4b6d1362bdfc6b985
Closes-Bug: #1785880

ansible/roles/neutron/templates/neutron-l3-agent-wrapper.sh.j2

@@ -2,12 +2,14 @@
 
 set -o errexit
 
+{% if enable_neutron_agent_ha | bool %}
 # NOTE(jeffrey4l): Remove all l3 related netns in case of multiple active routers in l3 high available mode.
 neutron-netns-cleanup \
         --config-file /etc/neutron/neutron.conf \
         --config-file /etc/neutron/l3_agent.ini \
         --config-file /etc/neutron/fwaas_driver.ini \
         --force --agent-type l3
+{% endif %}
 
 neutron-l3-agent \
         --config-file /etc/neutron/neutron.conf \