Add support for HWKMETRICS-420

[pilhuhn]

See https://issues.jboss.org/browse/HWKMETRICS-420

This adds 'admin' availability type. Also sub-state may be represented via tags on the data points, the client needs to be able to retrieve them as well.

Also the availability bucket query now returns that new field + 'unknown' See
hawkular/hawkular-metrics#522

This will only work with metrics 0.16+ (needs to be mentioned in RDoc)

[pilhuhn]

cc @yaacov