T938: do not bind DHCP relay to default port if unspecified

Binding isc-dhcp-relay to its default port (67 e.g. for IPv4) will
result in an error when starting up the service:

bad:
----
$ dhcrelay -q -4 -p 67 -c 10 -A 576 -m discard -i eth0.21 -i eth0 10.253.253.1
binding to user-specified port 67

good:
-----
$ dhcrelay -q -4 -c 10 -A 576 -m discard -i eth0.21 -i eth0 10.253.253.1

Setting removed from the IPv6 implementation, too!

1 files changed, 2 insertions, 2 deletions

diff --git a/src/conf_mode/dhcp_relay.py b/src/conf_mode/dhcp_relay.py
index f757491e0..61b494b7e 100755
--- a/src/conf_mode/dhcp_relay.py
+++ b/src/conf_mode/dhcp_relay.py
@@ -43,14 +43,14 @@ SERVERS="{{ server | join(' ') }}"
 INTERFACES="{{ interface | join(' ') }}"
 
 # Additional options that are passed to the DHCP relay daemon?
-OPTIONS="-4 -p {{ port }} {{ options | join(' ') }}"
+OPTIONS="-4 {% if port -%} -p {{ port }}{%- endif %} {{ options | join(' ') }}"
 """
 
 default_config_data = {
     'interface': [],
     'server': [],
     'options': [],
-    'port': '67',
+    'port': '',
     'hop_count': '10',
     'relay_agent_packets': 'forward'
 }