This makes it easy for a Fabric script to populate an env.roledefs

based on tagged EC2 instances.
A Fabric script just needs to import fabazon.roles.EC2RoleDefs and

assign an instance to env.roledefs, passing in the list of regions,

any role names, an optional tag attribute name, and an optional

dictionary of tags that each matched entry must have set.
The list of hosts will be looked up on demand, saving API requests and

keeping the Fabric script pretty quick.

Tested with a Fabric file, and saw it fetch the correct list of hostnames

on demand based on what was tagged in our list of EC2 instances.