Rename ListNetworks to Subnets
Fix openstack provider
Comment correction
Merge pull request #1168 from voidspace/rename-listnetworks

Rename environs.Environ.ListNetworks to Subnets

The ListNetworks method on the environs.Environ interface really returns info about subnets, not "networks". Method renamed appropriately.

(Review request: https://reviews.vapour.ws/r/479/)