feat(engine): add <plugins ...> to configuration

- add possibility to configure process engine plugins in bpm-platform.xml
  and processes.xml

related to #CAM-978
feat(ldap): add ldap library and config to distros

- add ldap library to all distributions
- make sure jboss subsystem instantiates process engine plugins

related to #CAM-978
Merge branch 'master' of github.com:camunda/camunda-bpm-platform

Conflicts:
	webapps/camunda-webapp/webapp/src/main/java/org/camunda/bpm/webapp/impl/security/auth/AuthenticationFilter.java