MGNSourceServer: disconnect before deleting

[fridim]

Source Servers must be disconnected from service before delete.

That change, if applied, fixes the issue:

An error occurred (ConflictException) when calling the DeleteSourceServer operation: Cannot delete source server s-xxxx. Source servers must not be actively replicating in order to be deleted.

[fridim]

@der-eismann ping?

[ekristen]

@fridim I could not cherry-pick your commit due to the divergence with the fork. However I've included your fix in this PR ekristen/aws-nuke#329, it will be merged later tonight.

If you have a chance, please check it out and let us know if you run into an issues by opening an issue over on the fork.

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