Allow publishing arbitrary headers via the "storage.objects.*.bytes"
counter

The change allow publishing arbitrary headers via the
"storage.objects.*.bytes" counter of the swift middleware.

The list of the headers to publish can be configured in the swift
middleware filter like this:

[filter:ceilometer]
use = egg:ceilometer#swift
metadata_headers = X_VAR1, x-var2

Change-Id: I26a4a573707a778c5b86d298840e7ae25a95deea
Merge "Allow publishing arbitrary headers via the "storage.objects.*.bytes" counter"