gw-core01: persist dhcp leases

/etc/dhcp.leases is not saved with `sysupgrade -b`

templates/gateways/etc/config/dhcp

@@ -12,7 +12,7 @@ config dnsmasq
 	option nonegcache '0'
 	option authoritative '1'
 	option readethers '1'
-	option leasefile '/tmp/dhcp.leases'
+	option leasefile '/etc/dhcp.leases'
 	option resolvfile '/tmp/resolv.conf.d/resolv.conf.auto'
 	option nonwildcard '1'
 	option localservice '1'