Replaced IpPrefix and IpAddress in the following methods
and cleanup related code:

 - Host.ipAddresses()
 - DefaultHost.ipAddresses()
 - HostDescription.ipAddress()
 - DefaultHostDescription.ipAddress()
 - HostService.getHostsByIp()
 - HostManager.getHostsByIp()
 - HostStore.getHosts()
 - GossipHostStore.getHosts()
 - SimpleHostStore.getHosts()
 - ProxyArpService.known()
 - ProxyArpManager.known()

As a result of the above cleanup, the "hosts" CLI command outputs
the IP addresses as "1.2.3.4" instead of "1.2.3.4/32".

Also, the following REST calls might be affected as well with
the above format replacement:
  - REST POST: config/topology
  - REST GET: topology/graph
diff --git a/core/api/src/main/java/org/onlab/onos/net/proxyarp/ProxyArpService.java b/core/api/src/main/java/org/onlab/onos/net/proxyarp/ProxyArpService.java
index 1e29a02..f49b5cf 100644
--- a/core/api/src/main/java/org/onlab/onos/net/proxyarp/ProxyArpService.java
+++ b/core/api/src/main/java/org/onlab/onos/net/proxyarp/ProxyArpService.java
@@ -3,7 +3,7 @@
 import org.onlab.onos.net.ConnectPoint;
 import org.onlab.onos.net.packet.PacketContext;
 import org.onlab.packet.Ethernet;
-import org.onlab.packet.IpPrefix;
+import org.onlab.packet.IpAddress;
 
 /**
  * Service for processing arp requests on behalf of applications.
@@ -17,7 +17,7 @@
      * @param addr a ip address
      * @return true if know, false otherwise
      */
-    boolean known(IpPrefix addr);
+    boolean known(IpAddress addr);
 
     /**
      * Sends a reply for a given request. If the host is not known then the arp