Implement Subnets method for ec2 provider

Fetch all the subnets known by the ec2 provider and return a SubnetInfo representing each one. 

This PR renames networks.BasicInfo to networks.SubnetInfo as it is a better reflection of its purpose. With the ec2 test server the error conditions of the Subnets methods are hard to test.

(Review request: http://reviews.vapour.ws/r/644/)