tests: Log network interfaces before and after destroy

[mxinden]

For better upstream Terraform debugging we would like to log the network
interfaces related to the VPC. For further details see:
hashicorp/terraform-provider-aws#1051

@erikkn This change will add something along the lines of this to the test log:

{
    "NetworkInterfaces": 
           ...
}

The goal would be to be able to analyse this with the Elastic search project. Should I log any particular keyword before or after so we can extract it from the logs?

[coreosbot]

Can one of the admins verify this patch?

[s-urbaniak]

I'll defer the actual implementation review to @alexsomesan, but if it does what it is supposed to do, I have no objection once green ;-)

[alexsomesan]

I'm afraid such debug sections will be forgotten around the code once the issues get fixed.
Do you think it makes sense to gate these sections with an environment variable flag so that they only run while that var is set? That way we can also grep the tests code for that var name to discover any stale debug statements.

[mxinden]

@alexsomesan Would a comment with a keyword like DEBUG be enough? This way we can grep for it in the code base.

[mxinden]

It fails on the second go, as in the best case there are no network interfaces no more. I will look into it.

[squat]

@alexsomesan @mxinden is this ready?

[mxinden]

@squat Added one small refactoring. I will request for review once ready.

@alexsomesan Any opinion on #2157 (comment) ?

[mxinden]

@squat @alexsomesan Could you give this another review?

[s-urbaniak]

@mxinden taking over, as INST-288 has been priorized

[mxinden]

ok to test

[mxinden]

@s-urbaniak @alexsomesan Tests are all green. Can you approve once again, so I can merge?