Make PIM hello interval configurable on a per-interface basis.

Change-Id: I7a0788be4445c7befbd947a3df893bcce1118bf5
diff --git a/apps/pim/src/main/java/org/onosproject/pim/impl/PimInterfaceConfig.java b/apps/pim/src/main/java/org/onosproject/pim/impl/PimInterfaceConfig.java
index 8d8ad78..48969c3 100644
--- a/apps/pim/src/main/java/org/onosproject/pim/impl/PimInterfaceConfig.java
+++ b/apps/pim/src/main/java/org/onosproject/pim/impl/PimInterfaceConfig.java
@@ -28,6 +28,7 @@
 
     private static final String INTERFACE_NAME = "interfaceName";
     private static final String ENABLED = "enabled";
+    private static final String HELLO_INTERVAL = "helloInterval";
     private static final String HOLD_TIME = "holdTime";
     private static final String PRIORITY = "priority";
     private static final String PROPAGATION_DELAY = "propagationDelay";
@@ -53,6 +54,18 @@
     }
 
     /**
+     * Gets the hello interval of the interface.
+     *
+     * @return hello interval
+     */
+    public Optional<Integer> getHelloInterval() {
+        if (node.path(HELLO_INTERVAL).isMissingNode()) {
+            return Optional.empty();
+        }
+        return Optional.of(Integer.parseInt(node.path(HELLO_INTERVAL).asText()));
+    }
+
+    /**
      * Gets the HELLO hold time of the interface.
      *
      * @return hold time