Make sure NetworkInterfaces and SubnetId updatable

We should make the properties 'NetworkInterfaces' and 'SubnetId'
updatable for AWS::EC2::Instance resource, so we can update the networks
for an instance.

Change-Id: I4a475fee6416696558807c189473a90d7517cfc0
Closes-Bug: #1280151
Merge "Make sure NetworkInterfaces and SubnetId updatable"