stable-1.15-1.6.0

• [dex-k8s-authenticator]: Now supports a kubectl credentials plugin for automatically managing identity tokens. Instructions for downloading the plugin and configuring kubectl can be found at https://<cluster-ip>/token/plugin. (#212, @jr0d)
• [cert-manager] usages is no longer definable as part of issuerRef, instead it is a key on its own (#196, @sebbrandt87)
• [elasticsearch] Fixes an issue that may cause the elasticsearch addon to fail to deploy. (#206, @branden)