[snmp] Fix a bug in SNMP alias mapping (#473)

* [snmp] Fix a bug in SNMP alias mapping Which was introduced in #448.

dockers/docker-snmp-sv2/alias_map.j2

@@ -1,5 +1,5 @@
 {
 {% for member in minigraph_ports.keys() %}
-  "{{member}}": "{{minigraph_ports[member]['alias']}}"
+  "{{member}}": "{{minigraph_ports[member]['alias']}}"{% if not loop.last %},{% endif %}
 {% endfor %}
 }

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

@@ -1,6 +1,7 @@
 import filecmp
 import os
 import subprocess
+import json
 
 from unittest import TestCase
 
@@ -21,6 +22,13 @@ def test_interfaces(self):
         argument = '-m "' + self.t0_minigraph + '" -p "' + self.t0_port_config + '" -t "' + interfaces_template + '"'
         output = self.run_script(argument) 
 
+    def test_alias_map(self):
+        alias_map_template = os.path.join(self.test_dir, '..', '..', '..', 'dockers', 'docker-snmp-sv2', 'alias_map.j2')
+        argument = '-m "' + self.t0_minigraph + '" -p "' + self.t0_port_config + '" -t "' + alias_map_template + '"'
+        output = self.run_script(argument)
+        data = json.loads(output)
+        self.assertEqual(data["Ethernet4"], "fortyGigE0/4")        
+
     def test_teamd(self):
         argument = '-m ' + self.t0_minigraph + ' -p ' + self.t0_port_config + ' -v "minigraph_portchannels.keys() | join(\' \')"'
         output = self.run_script(argument) # Mock the output via config.sh in docker-teamd