Rename plugin terminology in jobs; "plugin" instead of "name"

[mimir-d]

After feedback from multiple parties, it's not clear that the "name" in steps, targets managers, etc has to match with a plugin name from codebase.
Should switch all "name" references to "plugin", like:

• "TargetManagerName": "targetlist" -> "TargetManagerPlugin": "targetlist"
• step {"name": "exec"} -> step {"plugin": "exec"}