Fix is_mgmt_vrf_enabled when MGMT_VRF_CONFIG is not present the confi…

…g DB (sonic-net#885)

Fix is_mgmt_vrf_enabled for the case where MGMT_VRF_CONFIG is not at all in the config DB.
This is the case where mgmt vrf is never configured. The function throws error at
File "/usr/lib/python2.7/dist-packages/show/main.py", line 651, in is_mgmt_vrf_enabled
mvrf_dict = json.loads(p.stdout.read())
Two show commands uses is_mgmt_vrf_enabled. "show mgmt-vrf" and "show ntp"
Both commands throw error if mgmt vrf is never configured

Co-authored-by: Bing Sun <Bing_Sun@dell.com>

show/main.py

@@ -636,16 +636,19 @@ def is_mgmt_vrf_enabled(ctx):
         cmd = 'sonic-cfggen -d --var-json "MGMT_VRF_CONFIG"'
 
         p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-        stdout = p.communicate()[0]
-        if p.returncode == 0:
-            mvrf_dict = json.loads(stdout)
-
-            # if the mgmtVrfEnabled attribute is configured, check the value
-            # and return True accordingly.
-            if 'mgmtVrfEnabled' in mvrf_dict['vrf_global']:
-                if (mvrf_dict['vrf_global']['mgmtVrfEnabled'] == "true"):
-                    #ManagementVRF is enabled. Return True.
-                    return True
+        try :
+            mvrf_dict = json.loads(p.stdout.read())
+        except ValueError:
+            print("MGMT_VRF_CONFIG is not present.")
+            return False
+
+        # if the mgmtVrfEnabled attribute is configured, check the value
+        # and return True accordingly.
+        if 'mgmtVrfEnabled' in mvrf_dict['vrf_global']:
+            if (mvrf_dict['vrf_global']['mgmtVrfEnabled'] == "true"):
+                #ManagementVRF is enabled. Return True.
+                return True
+
     return False
 
 #