Waldur-core

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- Added project_group field to project logging.

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- Bugfixes.
- Support billing data extraction from nova.

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- Alert API filtering extensions.
- Bugfixes of PaaS instance monitoring polling.

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- Extend alert filtering API.
- Bugfixes.

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- Alert filterting and statistics bugfixes.
- Support for application-specific Zabbix templates/checks.
- Alert endpoint for creating alerts with push.

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- Support for authentication token passing via query parameters.
- Alert API: historical and statistical.
- Support for historical quota usage/limit data via Zabbix backend.
- Filtering and minor API modifications across multiple endpoints.