AWS: Allow configuration of a single security group for ELBs

[nbutton23]

What this PR does / why we need it:
AWS has a hard limit on the number of Security Groups (500). Right now every time an ELB is created Kubernetes is creating a new Security Group. This allows for specifying a Security Group to use for all ELBS

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:
For some reason the Diff tool makes this look like it was way more changes than it really was.
Release note:

[k8s-ci-robot]

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[k8s-reviewable]

This change is 

[nbutton23]

@justinsb You asked me to tag you in this PR

[justinsb]

@k8s-bot ok to test

/lgtm

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justinsb, nbutton23

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[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 36721, 46483, 45500, 46724, 46036)

[dlsniper]

Just a small comment about an inaccuracy from the PR. AWS has a default, soft-limit, of 500 security groups. While I'm sure there's a hard limit somewhere, I couldn't find the docs for it.

Here's the link to the documentation regarding this: http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Appendix_Limits.html#vpc-limits-security-groups

[nbutton23]

@dlsniper Interesting. . . Our AWS concierge told us it was a hard limit for performance reasons. . .