Make parentModule POM build plugins overridable

[sdruskat]

All modules inheriting from parentModule need this file in /src/findbugs/, inherited from parentModule. Modules that do not wish to use findbugs shuoldn't have to have it.

[sdruskat]

This is a general problem, e.g., I've tried to use a different release mechanism (via https://bitbucket.org/atlassian/jgit-flow/) and it failed miserable, because the deploy goal is hit twice, so the release is staged twice, and will fail for the second time because the version has already been released to Maven Central.

This makes module development a pain!

parentModule should define <pluginManagement> if needs be, but should leave everything else to its children. This would also get rid of the Findbugs issue mentioned above. All these are hidden requirements which not everyone should have to fulfill.