Use resource discovery for Network Services

This change migrates the network services pollsters to
leverage resource discovery through pipeline yaml. This
will also be more HA friendly to distribute work across
multiple central agents.

Change-Id: I4d454e98974438438c166051451b76ce9fbbc2a4
Add support for metering VPNaaS

Change-Id: Ida9e04b13d54ab4853a4ba30d2148ac7bc8ba244
Partially-Implements: blueprint ceilometer-meter-vpnaas
Multi meter arithmetic transformer

New pipeline transformer that enables us to perform arithmetic calculations
over one more meters and/or their metadata, for example:

    memory_util = 100 * memory.usage / memory .

The calculation is limited to meters with the same interval.

Example configuration:

    - name: "arithmetic"
      parameters:
        target:
          name: "memory_util"
          unit: "%"
          type: "gauge"
          expr: "100 * $(memory.usage) / $(memory)"

To demonstrate the use of metadata, here is the implementation of
a silly metric that shows average CPU time per core::

    - name: "arithmetic"
      parameters:
        target:
          name: "avg_cpu_per_core"
          unit: "ns"
          type: "cumulative"
          expr: "$(cpu) / ($(cpu).resource_metadata.cpu_number or 1)"

Expression evaluation gracefully handles NaNs and exceptions. In such
a case it does not create a new sample but only logs a warning.

DocImpact: add documentation about using this new transformer
Implements: blueprint arithmetic-transformer
Change-Id: I1b637e5b1d1bb15ed3c3d7ec758d2a684eaccf21
Adds Content-Type to alarm REST notifier

When doing a POST request in the REST alarm notifier,
we should include a Content-Type: 'application/json' header.

Closes-Bug: #1346147
Change-Id: Ic02c8caeb294763443858dbbb3192e09c594c8cf
Merge "Adds Content-Type to alarm REST notifier"
Support for metering FWaaS

Partially-Implements: blueprint ceilometer-meter-fwaas

Change-Id: I3b0e40fa3bbddc304d3adf936626334342288a2d
Merge "Use resource discovery for Network Services"
Merge "Add support for metering VPNaaS"
Merge "Support for metering FWaaS"