Add support for storing subnet id in ec2instance and report vpc for stored subnet id in describeInstances

[KraProgrammer]

Add support for persisting subnet id. The subnet id is also used to derive the vpc id. Previously the default subnet id and vpc id were returned for all instances.

[maxiaohao]

Hi @KraProgrammer really sorry for the late response. I note you've closed the PR but if you would reopen it I think we are not far away from merging this great feature! Thanks.

[nottoseethesun]

I'm planning on having this work merged in.