Actually allow forkers to sign their own releases

* If you run an internal repo, you may be required to have signed artifacts
  So to make their life easier, use a property to define the default signing key
  Allows for others to swap in the key they use via the -Darguments=-Dgpg.keyname=
  trick
Move the javadoc configuration to the default.

* Making the configuration of javadoc tied to the execution makes it harder for
  people to tweak the javadoc configuration as they have to invoke the lifecycle
  rather than just invoke 'mvn javadoc:javadoc' to see the effects quickly