Spec for adding aggregate support for Datalog

Datalog currently does not support using aggregates such as
COUNT, SUM, AVERAGE etc. These are required to write more
real policies involving aggregate queries like number of
ports in a subnet, number of VMs in a host, Average CPU
utilization, Average number of bytes etc..

Change-Id: I98aac7c434db759e25e22d02c0fec22d131fe59f
Merge "Spec for adding aggregate support for Datalog"