Update jordan/icinga2 Docker tag to v2.15.0

This MR contains the following updates:

Package	Type	Update	Change
jordan/icinga2	final	minor	2.14.3 -> 2.15.0
jordan/icinga2	final	minor	2.14.0 -> 2.15.0

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testserver/docker_icinga/templates/Dockerfile

-FROM jordan/icinga2:2.14.0
+FROM jordan/icinga2:2.15.0
 
 # Install additional Packages 
 RUN apt-get update \