fix(osgi): Fix generation of MANIFEST.MF

- move configuration for maven-bundle-plugin from engine-osgi to root POM
- change property "camunda.artifact" to "org.camunda.bpm" for engine POM
  because it has classes in that package that need to be exported
- declare org.jboss.vfs and the javax-packages servlet, transaction, ejb,
  mail and xml, catalina and commons.lang as optional imports for engine

Closes #CAM-1551
Merge branch 'master' of github.com:camunda/camunda-bpm-platform