CORD-1901 Invalid /etc/network/interfaces

Change-Id: If4df27de34f39e4a67a35b52095cb8ef68218c89
diff --git a/library/netfile.py b/library/netfile.py
index bb37f43..5bde0ec 100755
--- a/library/netfile.py
+++ b/library/netfile.py
@@ -241,7 +241,7 @@
 
 # which attributes should be ignored and not be written as single
 # attributes values against and interface
-write_ignore = ["auto", "type", "config", "description"]
+write_ignore = ["auto", "type", "config", "description", "source"]
 
 # specifies the order in which attributes are written against an
 # interface. Any attribute note in this list is sorted by default