Added instance hours report

Initial version of report to calculate unit hours used
for nova instances

Breakdown by flavor, flavor class, account/billing types and by tenant.
Add tenant info cache. Refactor Instance hr report.

Added cache table for basic tenant info for reports.
Refactor instance_hours report to use table.
Improve performance of tenant info update.

use bulk sql operations to speed up the tenant info update, as it's
taking  ~40s/1000 tenants to update on a decent machine.
Fix some tests broken by rebase.

Fix unittests broken by rebase.
Also, renumber migration due to collision.
Add Apache license header to new files.
Merge pull request #315 from DragonDM/instance_hours_report

Instance hours report