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/host/HostService.java b/core/api/src/main/java/org/onlab/onos/net/host/HostService.java
index 09034eb..f18569f 100644
--- a/core/api/src/main/java/org/onlab/onos/net/host/HostService.java
+++ b/core/api/src/main/java/org/onlab/onos/net/host/HostService.java
@@ -7,7 +7,6 @@
 import org.onlab.onos.net.Host;
 import org.onlab.onos.net.HostId;
 import org.onlab.packet.IpAddress;
-import org.onlab.packet.IpPrefix;
 import org.onlab.packet.MacAddress;
 import org.onlab.packet.VlanId;
 
@@ -60,7 +59,7 @@
      * @param ip ip address
      * @return set of hosts with the given IP
      */
-    Set<Host> getHostsByIp(IpPrefix ip);
+    Set<Host> getHostsByIp(IpAddress ip);
 
     // TODO: consider adding Host getHostByIp(IpAddress ip, VlanId vlan);