Detach ENI before deleting

[uthark]

Fix for issue #537.

Try to detach ENI before deleting it.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mogren]

Thanks a lot @uthark for catching this. There are a lot of corner cases where throttling can put the data store in a bad state.

[uthark]

@mogren Is there any plans to release a minor 1.5.1 just with the bugfixes or do you plan to release 1.6.0 only?

[mogren]

@uthark I would really like to get #371 merged first, just need a little more testing. So far everything looks good.

The top priority is to resolve #282, #330 and #525, which are all basically the same issue. So once we get those fixed I'll make a v1.6-rc build and start some more deep testing.

There is some work to get a new version out, but I'd prefer if we could be more frequent with new releases instead of trying to put too many things in before cutting the next one.