Allow customized reseller_prefix in Ceilometer middleware for Swift

Swift can be configured to use a different reseller prefix than AUTH_.
In this case Ceilometer returns None as resource_id.

This patch allows to set the reseller prefix, with a fallback to AUTH_
if not set. It also ensures that the reseller prefix ends with ``_``.
Two additional tests to verify this were added.

Change-Id: I1ba40091582da46c0385a71d1828bb3820a5e2b5
Closes-Bug: #1260965
Merge "Allow customized reseller_prefix in Ceilometer middleware for Swift"