Add new rate-based disk and network pipelines

Addresses: BP rates-derived-from-cumulative

Add new pipeline config to derive rate-based gauge meters
from the current set of cumulative disk and network meters
gathered by the compute agent.

The latter form of meter is not suitable for alarming
against a static threshold, whereas rate-based meters would
allow for example autoscaling actions to be driven off the
observed per-second rates of I/O.

Change-Id: I5764797d4eebb129b66a1524d47c38282c6ef095
Merge "Add new rate-based disk and network pipelines"