Add easy population of roledefs from tagged EC2 instances.

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.