Waldur-core

---------------
- Don't use urllib3 1.18 because it's not compatible with old setuptools.

---------------
- Introduction of usage-based price estimatation and billing.
- Prohibit editting of price estimates manually.
- Preserve consumption details of resources.
- Refactor price estimates calculation for link and unlink operations.
- Remove "resource provisioned" signal.
- Add admin action for recalculation of price estimates.
- Display resource consumption details in admin.
- Introduce base class for background tasks.

---------------
- Bumped django-model-utils dependency version to 2.5.2.

---------------
- Migrated to Django 1.8 UUIDField.

---------------
- Validate UUID in filters.

---------------
- Fix scoped service settings descendant resource unlinking.
- Enable HTTP client debugging.