add a helper switchd ifupdown script that returns reserved vlan range

Ticket:
Reviewed By: CCR-2279
Testing Done: tested ifupdown2 reserved vlan checks

THis patch adds a switchd ifupdown script to return the reserved vlan.

Ifupdown will use this to warn the user when he uses a reserved vlan

(THis patch is a result of wilsons request to have a reserved vlan check
if ifupdown)
(cherry picked from commit 4e693849aba1f1821dba9cfdc567c92baeb23db5)

ifupdownaddons/modulebase.py

@@ -4,6 +4,7 @@
 # Author: Roopa Prabhu, roopa@cumulusnetworks.com
 #
 
+import os
 import re
 import io
 import logging
@@ -324,11 +325,14 @@ class moduleBase(object):
     def _get_reserved_vlan_range(self):
         start = end = 0
         get_resvvlan = '/usr/share/python-ifupdown2/get_reserved_vlan_range.sh'
+        if not os.path.exists(get_resvvlan):
+            return (start, end)
         try:
             (s, e) = self.exec_command(get_resvvlan).strip('\n').split('-')
             start = int(s)
             end = int(e)
-        except:
+        except Exception, e:
+            self.logger.debug('%s failed (%s)' %(get_resvvlan, str(e)))
             # ignore errors
             pass
         return (start, end)