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/net/src/main/java/org/onlab/onos/net/host/impl/HostManager.java b/core/net/src/main/java/org/onlab/onos/net/host/impl/HostManager.java
index 4f9bcbb..e4d158d 100644
--- a/core/net/src/main/java/org/onlab/onos/net/host/impl/HostManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/host/impl/HostManager.java
@@ -33,7 +33,6 @@
 import org.onlab.onos.net.provider.AbstractProviderRegistry;
 import org.onlab.onos.net.provider.AbstractProviderService;
 import org.onlab.packet.IpAddress;
-import org.onlab.packet.IpPrefix;
 import org.onlab.packet.MacAddress;
 import org.onlab.packet.VlanId;
 import org.slf4j.Logger;
@@ -121,7 +120,7 @@
     }
 
     @Override
-    public Set<Host> getHostsByIp(IpPrefix ip) {
+    public Set<Host> getHostsByIp(IpAddress ip) {
         checkNotNull(ip, "IP address cannot be null");
         return store.getHosts(ip);
     }