check_mode should behave pretty similarly to non-check mode -
just don't actually create or delete subnets or change tags.
Using DryRun for check_mode behaves very differently and results
in the following module failure:
```
"msg": "Unable to update tags for subnet-abcd1234,
error: EC2ResponseError: 412 Precondition Failed
<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<Response><Errors><Error><Code>DryRunOperation</Code>
<Message>Request would have succeeded, but DryRun flag
is set.</Message></Error></Errors>
<RequestID>12345678-abcd-1234-abcd-abcd1234abcd</RequestID></Response>"
```
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