Please document the determine_subnet() method in ec2_instance provider

[prozach]

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I find this method confusing. Also, I think its causing a bug in config_with_network_details method upon security group detection. Security groups have a group_name method, but we only seem to be searching for the 'Name' tag. Documentation about what determine_subnet() is intended to do and return would help fix the bug and improve the docs. @garethr or @DavidS I expect can help here. Pretty please.