Stored metrics into compute_nodes as a json dictionary

The compute manager uses plugins to get the metrics information of each node.
And then sends and stores in DB for the scheduler to read and do scheduling.

The patch reuses table compute_nodes and adds a new TEXT column to store the
metric data as a json dictionary.

This is part of the blueprint utilization-aware-scheduling and the
blueprint generic-host-state-for-scheduler.

Change-Id: I4d20565567e9907cfcc92b1c1b15f3b35a806797
Signed-off-by: Shane Wang <shane.wang@intel.com>
Merge "Stored metrics into compute_nodes as a json dictionary"