Support for existing IAM roles

[eytan-avisror]

fixes #56

This implements additional configuration for the EKSCF API called roleName which will override the creation of a new IAM role within the cloudformation stack and instead use an existing role for the instance profile + bootstrapping.

This helps with the use case where a user does not wish to create IAM roles from within the cluster and can now provide an externally created IAM role.

In this case the IAM role provided must have the basic minimum roles in order to join the cluster, and managedPolicies field is not supported as the role is now essentially managed by the user and not instance-manager.

Added unit tests

[codecov]

Codecov Report

Merging #62 into master will increase coverage by 0.11%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   72.64%   72.76%   +0.11%     
==========================================
  Files           6        6              
  Lines         797      815      +18     
==========================================
+ Hits          579      593      +14     
- Misses        160      162       +2     
- Partials       58       60       +2

Impacted Files	Coverage Δ
...rovisioners/ekscloudformation/ekscloudformation.go	82.44% <100%> (+0.76%)	⬆️
...ollers/provisioners/ekscloudformation/bootstrap.go	81.81% <20%> (-6.42%)	⬇️

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[kevdowney]

LGTM 👍