hosttracker maintains state in memory (map) which would not work in clustered env

hosttracker maintains state in memory (map) which would not work in clustered environment.

In following class
org.opendaylight.l2switch.hosttracker.plugin.internal.HostTrackerImpl

it maintains:
private ConcurrentHashMap<HostId, Host> hosts;

current implementation may not work as desired in clustered env with l2switch deployed on multiple nodes(controller instances).