Name and unit mapping for rate_of_change transformer

Precursor-to: BP rates-derived-from-cumulative

Adds support for regex-based name and unit mapping to the
rate_of_change transformer. Regular expression back-references
are key to simplifying potentially repetitive config.

This avoids a explosion of pipelines to apply conversions to
classes of similar meters. For example to allow the following
sample derivations:

  * disk.read.requests (request) -> disk.read.requests.rate (request/s)
  * disk.write.requests (request) -> disk.write.requests.rate (request/s)
  * disk.read.bytes (B) -> disk.read.bytes.rate (B/s)
  * disk.write.bytes (B) -> disk.write.bytes.rate (B/s)

to be mapped via a single transformer config:

    transformers:
        - name: "rate_of_change"
          parameters:
              source:
                  map_from:
                      name: "disk\\.(read|write)\\.(bytes|requests)"
                      unit: "(B|request)"
              target:
                  map_to:
                      name: "disk.\\1.\\2.rate"
                      unit: "\\1/s"
                  type: "gauge"

Change-Id: I94b6bfbef538a8b067c3c5ff65066505a199bc5a
Merge "Name and unit mapping for rate_of_change transformer"