Add extra_resources field to compute_nodes table

The compute_nodes table has informtion about allocation and
availability of a predefined set of resources (e.g. memory,
disk, vcpu). The extra_resources field will contain a json
string of arbirtrary information describing an extensible set
of resources that will be tracked by resource tracker plug-ins.

This differs from other fields containing json strings that have
been added recently, including pci_stats and metrics. In particular,
pci_stats is used for information about pci devices, whereas metrics
contains continuously monitored usage data and is updated at different
times to extra_resources and according to a different trigger.
Accordingly, it makes sense to seperate these into different fields.

This is part of: blueprint extensible-resource-tracking

Change-Id: I75e339c2a2827875078c937e57c37e85d3ad1852
Merge "Add extra_resources field to compute_nodes table"