Improve logging for retain plugins

[rhierlmeier]

With this PR I improved the logging of the retainsPlugins processing. It produces also a warning when in the retainPlugin-list and unknown execution is specified. Helps to avoid configuration errors.

In additional I changed the expected format for the retainsPlugins elements. It is now :<artifactId>[@executionId] according to the Maven standards. The old format :<artifactId>[:executionId] is normally targeting a Maven goal and not an execution.

[glimmerveen]

Good suggestion to change the syntax to better align with the Maven notation conventions.

Just a small remark on the check to avoid configuration errors: it will result in "false warnings" when someone wants to rely on the inheritance of configuration: when one wants to configure retainPlugin in a parent pom, that only applies to some of the childs, the other child modules would produce a warning. This can of course can be addressed by only configuring this retainPlugin on the level where the plugins are actually executed, but that would result in some duplication of configuration.