Billing statistics improved

Instance tracking information was improved to contain additional
information on instances such as type name, descriptive title
and number of VMs used by the application.

Added ability to retrieve non-aggregated statistics.
Also instance statistics API refactoring was made.

Implements blueprint app-catalog-billing

Change-Id: I8f3ea1a3947ceaf7deb2ee62b4ad41cbf230596e
Merge "Billing statistics improved"