Cut 2.15.0 release. (#6509)

Signed-off-by: NBartek Plotka <bwplotka@gmail.com>

CHANGELOG.md

-## 2.15.0-rc.0 / 2019-12-18
+## 2.15.0 / 2019-12-23
 
 * [CHANGE] Discovery: Removed `prometheus_sd_kubernetes_cache_*` metrics. Additionally `prometheus_sd_kubernetes_workqueue_latency_seconds` and `prometheus_sd_kubernetes_workqueue_work_duration_seconds` metrics now show correct values in seconds. #6393
 * [CHANGE] Remote write: Changed `query` label on `prometheus_remote_storage_*` metrics to `remote_name` and `url`. #6043

VERSION

-2.15.0-rc.0
+2.15.0