M101 - add description to mission exec abort events
M101 - Add descriptions to mission exec events

- also clean up the code a bit
Merge pull request #2138 from Bobfrat/abort_events

M101 - add description to mission executive events
some clean-up and comment update
include provider_id (from org_obj._id) and actor_id in configuration
IMS: pass agent_id from context to the configuration in start_platform_agent_instance
pass get_system_actor_header as headers argument in IMS.start_platform_agent_instance and IMS.start_instrument_agent_instance calls
initial version of exclusive access for mission execution
minor typo fix (no change in functionality)
minor typo fix