WIP: Initial support for custom SSM paths and IAM Roles

[preflightsiren]

Wanted to share some initial development and background. This adds support for two annotations to control the custom SSM endpoint, and IAM Role assumption.

apiVersion: instancemgr.keikoproj.io/v1alpha1
kind: InstanceGroup
metadata:
  annotations:
    instancemgr.keikoproj.io/ssm-iam-role: "arn:aws:iam::123456789:role/my-custom-role"
    instancemgr.keikoproj.io/ssm-ami-path: "/my/custom/ami/param"

This uses the aws helper stscreds to get new credentials. Need to test the credential refresh. It's quite ugly using a global shared session - open to suggestions how this might work better.

[codecov[bot]]

Codecov Report

Merging #336 (4dffa60) into master (5215605) will decrease coverage by 0.23%. The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   51.08%   50.84%   -0.24%     
==========================================
  Files          33       33              
  Lines        4504     4531      +27     
==========================================
+ Hits         2301     2304       +3     
- Misses       2062     2084      +22     
- Partials      141      143       +2     

Impacted Files	Coverage Δ
controllers/providers/aws/ssm.go	0.00% <0.00%> (ø)
controllers/provisioners/eks/eks.go	87.23% <ø> (ø)
controllers/provisioners/eks/helpers.go	90.82% <44.44%> (-0.58%)	:arrow_down:

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