Fix l3_agent.ini rendering error when DVR

The if/else blocks in this file cause a rendering error on control nodes
when enable_neutron_dvr: yes

ParsingError: File contains parsing errors: <???>
        [line  3]: u'    external_network_bridge =\n'

Change-Id: Ia461dcbbec531c4c6295b3c7e10da12c57b7d58b
Closes-Bug: 1626995

ansible/roles/neutron/templates/l3_agent.ini.j2

-# l3_agent.ini
+#jinja2: trim_blocks: False
 [DEFAULT]
 {% if enable_neutron_dvr | bool %}
     {% if inventory_hostname in groups['network'] %}