Keep parent in `flatten-bom` configuration

[vy]

flatten-bom configuration of flatten-maven-plugin doesn't keep the parent. This causes build failures for artifacts whose dependencies need to be resolved at runtime. Consider the following problem experienced in log4j-changelog-maven-plugin:

1. log4j-changelog-maven-plugin depends on log4j-changelog
2. log4j-changelog inherits from log4j-tools-parent
3. log4j-tools-parent inherits from log4j-tools-bom
4. log4j-tools-bom inherits from logging-parent
5. logging-parent contains dependencyManagement, etc. entries that are used by log4j-changelog-maven-plugin
6. Dependencies of log4j-changelog-maven-plugin et al. is resolved at runtime
7. Though at runtime, the deployed log4j-tools-bom is used, which is flattened and hence doesn't have a parent!
8. Hence, at runtime, log4j-transform-maven-plugin execution fails due to missing logic that should have been inherited from logging-parent

To avoid this, flatten-bom configuration should better keep the parent using <parent>keep</parent> in pomElements block.

[vy]

Fixed in 041768f.

[vy]

Reproduction of the issue:

$ git clone https://github.com/apache/logging-log4j-tools.git && cd $_
$ git checkout 18425d95a4d150de905929d55bf55545d70f3d91
$ ./mvnw -N log4j-changelog:export@export-changelog -X
...
[INFO] --- log4j-changelog:0.5.0:export (export-changelog) @ log4j-tools-bom ---
[WARNING] The POM for org.apache.logging.log4j:log4j-changelog:jar:0.5.0 is invalid, transitive dependencies (if any) will not be available: 2 problems were encountered while building the effective model for org.apache.logging.log4j:log4j-changelog:0.5.0
[ERROR] 'dependencies.dependency.version' for org.osgi:osgi.annotation:jar is missing. @ 
[ERROR] 'dependencies.dependency.version' for com.github.spotbugs:spotbugs-annotations:jar is missing. @