use other defaults for perModuleEmail and snapshotDependencies

I think perModuleEmail and snapshotDependencies should have other defaults. They are currently enabled by default, which are the defaults set by the Maven plugin when creating a new job. But in the context of the DSL it is strange to specify an element to remove some feature. So I changed the defaults, so that both elements need to be specified to enable these features. I will change the wiki in case you merge this.