M101: create mission manager when platform driver in place

Create and destroy mission manager in _initialize_this_platform and _reset_this_platform. Creation in particular is now done once there's a platform driver in place to allow mission executive to inspect attributes associated to the driver (eg., commands) in a more generic way. 

Also, destruction of mission manager (when _reset_this_platform is called) consists of aborting all ongoing mission executions, if any.

@edwardhunter please merge

@bobfrat please review