More flexible platform command dispatch involving child instruments

See https://confluence.oceanobservatories.org/display/CIDev/Platform+Agent+adjustments+for+Mission+Execution

All relevant unit and integration tests completing OK locally (including Bob's TestSimpleMission.test_simple_simulator_mission)

Also, includes a fix to a broken test_capabilities test (due to missing 'mission' param in platform agent schema).

@edwardhunter please review/merge
@Bobfrat  please review