Add examples and script to generate sample interfaces file

Ticket: CM-2643
Reviewed By:
Testing Done: Tested build/install

This also pulls in python-gvgen package from wheezy sid into our
upstream dir. Previously i had packaged the gvgen module directly
into the ifupdown package

docs/examples/interfaces_with_template

@@ -0,0 +1,26 @@
+#
+# Example interfaces file using mako templates
+#
+# The below section can be copied into
+# /etc/network/interfaces file 
+# or
+# to a file under /etc/network/interfaces.d/
+# and include in the interfaces file using the
+# 'source' command.
+#
+# see manpage interfaces(5) for details
+# 
+#
+
+%for v in range(1000,1100):
+auto vlan-${v}
+iface vlan-${v} inet static
+    bridge-ports glob swp1-6.${v}
+    bridge-stp on
+    bridge-treeprio 32768
+    bridge-ageing 200
+    bridge-maxhops 10
+    bridge-maxage 10
+    bridge-fdelay 10
+%endfor
+