[master] fixed interface-config type (#3716)

src/bin/keactrl/kea.conf.pre

@@ -9,7 +9,7 @@
 "Dhcp4":
 {
 # Add names of interfaces to listen on.
-  "interfaces-config:" {
+  "interfaces-config": {
     "interfaces": [ ]
   },
 
@@ -36,7 +36,7 @@
 "Dhcp6":
 {
 # Add names of interfaces to listen on.
-  "interfaces-config:" {
+  "interfaces-config": {
     "interfaces": [ ]
   },