When there are multiple untagged security groups, a security group may
fail deletion with DependencyViolation.  Before this commit, that
aborted that round of deleteEC2VPC, and when the blocking dependency
was another untagged security group, deletion would hang [1].

With this commit, we keep iterating through all untagged security
groups (just like we do for tagged security groups).  For security
group cycles, deletion will fail for the first few groups while we
drain out their ingress and egress rules.  But deletion will succeed
for at least some of the cycle.  And the rule draining will have
completely deconstructed the cycle, so the second pass through will
clear out all the security groups.

So prior to 55630a3 (pkg/destroy/aws: Delete security groups by
VPC, 2019-08-13, openshift#2214), destroy would hang on *any* untagged security
group.  And prior to this commit, it would hang when two untagged
security groups referenced each other (or, if we got unlucky, A
referenced B and our DescribeSecurityGroupsPages call always happened
to return B first).  With this commit, we should be good to go.

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1747519#c5