chore(osgi/engine): Remove "Activiti" from OSGi-Module and pom.xml

- remove "activiti" from log-messages
- change directory to search for process definitions to OSGI-INF/processes and add constant
- replace "OSGI-INF/processes" with constant
- change BUNDLE_ACTIVITI_HEADER to BUNDLE_PROCESS_DEFINITIONS_HEADER and set its value to "Process-Definitions"

related to CAM-1538