posixusers_baseline.py: Handle users with nonexistent primary groups
GroupLogic: sort after other plugins so GroupLogic can use, e.g., Probe groups
GroupLogic: avoid infinite recursion when using metadata.query in GroupLogic
docs: clarify when JSON/YAML Properties files were added
New plugin: AWSTags

AWSTags allows querying tags from EC2, and setting groups based on the
tag names or values.
Metadata: better logging when updating XML data fails