Require a list of plugins (not just one).

[tskluzac]

Currently 1 activity requires 1 plugin. This is fine for our existing use cases, but if we get something more complicated (e.g., I want to be able to run SHELL and PYTHON), then we will need this capability.

Should be a largely easy fix that mainly touches the campaign.py and messagehandler.py (instead of checking if one plugin is present, check a tree of them).