changed syntax of :pom-plugins definition for pom task

1. support of [:configuration ... ] was removed
2. it was replaced with map which contains arbitrary XML definitions,
syntax is the same as in pom-additions

3. also added use case in pom-plugins
task pom / :pom-plugins
added backward compatibility with version 2.3.3
task pom / :pom-plugins - bug fixed: extra <configuration> tag
More nuanced merging of project defaults.
Merge pull request #1342 from dominikdz/master

changed syntax of :pom-plugins definition for pom task
Merge pull request #1341 from llasram/meta-merge-defaults

More nuanced merging of project defaults.