remove unused create_command parameter in _instruments_execute_agent method
more graceful platform agent command dispatch involving child instruments

https://confluence.oceanobservatories.org/display/CIDev/Platform+Agent+adjustments+for+Mission+Execution
more flexible shutdown logic in test_platform_agent_with_rsn; and fix broken test_capabilities  (due to missing param 'mission' in scheme)
Merge pull request #2057 from carueda/pa_child_state_flex

More flexible platform command dispatch involving child instruments