fix: generation of MANIFEST

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,
and xml as optional imports for engine
set camunda.artifact property for engine-spring
add maven-bundle-plugin and maven-jar-plugin to engine-spring module

Closes #1551