[L2 switch mode] Update l2switch.j2 template (#5981)

- Why I did it
The l2switch.j2 template does not include all fields for PORT. This could be incompatible with the 201911 image or later.

- How I did it
Update l2switch.j2 template and add a unit test.

src/sonic-config-engine/data/l2switch.j2

@@ -2,13 +2,16 @@
     "DEVICE_METADATA": {{ DEVICE_METADATA | tojson }},
     {% set ns = {'firstPrinted': False} -%}
     "PORT": {
-        {%- for key,value in PORT.iteritems() -%}
+        {%- for key, value in PORT.items() -%}
         {%- if ns.firstPrinted %},{% endif %}
 
         "{{ key }}": {
-            "alias": "{{ value.alias }}",
+            {%- for keyPort, valuePort in value.items() %}
-            "lanes": "{{ value.lanes }}",
+
-            "admin_status": "up"
+            {% if keyPort != "admin_status" %}"{{ keyPort }}": "{{ valuePort }}",{% endif %}
+            {%- endfor %}
+
+            "admin_status": "{{ value.admin_status|default("up") }}"
         }
         {%- if ns.update({'firstPrinted': True}) %}{% endif -%}
         {%- endfor %}

src/sonic-config-engine/tests/test_j2files.py

@@ -99,6 +99,13 @@ class TestJ2Files(TestCase):
 
         self.assertTrue(json.dumps(sample_output_json, sort_keys=True) == json.dumps(output_json, sort_keys=True))
 
+        template_dir = os.path.join(self.test_dir, '..', 'data', 'l2switch.j2')
+        argument = '-t ' + template_dir + ' -k Mellanox-SN2700 -p ' + self.t0_port_config
+        output = self.run_script(argument)
+        output_json = json.loads(output)
+
+        self.assertTrue(json.dumps(sample_output_json, sort_keys=True) == json.dumps(output_json, sort_keys=True))
+
     def test_qos_arista7050_render_template(self):
         arista_dir_path = os.path.join(self.test_dir, '..', '..', '..', 'device', 'arista', 'x86_64-arista_7050_qx32s', 'Arista-7050-QX-32S')
         qos_file = os.path.join(arista_dir_path, 'qos.json.j2')