Map a mission to a specific agent

[JingfeiPeng]

Is your feature request related to a problem? Please describe.
Currently missions in scenario are mapped based on an order to agents in example files

Describe the solution you'd like
We need to figure out a way for direct mapping of mission to agents. 1 possible solution is to specify the mission ids in scenario and make example code import missions in scenario and map its agents to that.

Describe alternatives you've considered
Specifying target agent ids in the mission definition in scenario. However this makes scenarios tightly coupled with examples. This is not a good approach

[saulfield]

@Gamenot is this fixed by #1868? Please reopen if not.