EC2 inventory management

This PR introduces updates to EC2.py as detailed in the commits. In summary:

 - INI file settings can all be overridden by environment labels
 - INI file search path is more extensive
 - Additional settings permitting tag/value mapping to host group names for said tag/value
 - Additional settings permitting un-wrapping of tag values into lists
 - Additional settings permitting additional filtering of hosts in inventory based on tag and value