dhcp: allow user to specify router IP

In BPFire HA deployment, a floating/shared router IP
is required for backend/endpoint server. by default
BPFire uses the primary IP on green0 when running
setup script. Now the floating/shared router IP can
be added to green0 interface as secondary IP through
loxilb UI, keepalived UI can configure the secondary
IP as virtual ipaddress, when HA failover happens,
keepalived will move the virtual ipaddress to new active
BPFire.

Signed-off-by: Vincent Li <vincent.mc.li@gmail.com>

langs/en/cgi-bin/en.pl

@@ -1596,6 +1596,7 @@
 'invalid secondary dns' => 'Invalid secondary DNS.',
 'invalid secondary ntp' => 'Invalid Secondary NTP server address',
 'invalid start address' => 'Invalid start address.',
+'invalid router address' => 'Invalid router address.',
 'invalid time entered' => 'Invalid time entered.',
 'invalid time period' => 'Invalid time period',
 'invalid uplink speed' => 'Invalid uplink speed.',
@@ -2450,6 +2451,7 @@
 'standard login script' => 'Standard login script',
 'start' => 'Start',
 'start address' => 'Start address:',
+'router address' => 'Router address:',
 'start ovpn server' => 'Start OpenVPN Server',
 'state or province' => 'State or Province',
 'static ip' => 'Static IP',