Support additional EKS config options

[richardcase]

/kind feature
/area provider/eks

Describe the solution you'd like
At present we only expose the --kubelet-extra-args argument to EKS bootstrap.sh via the EKSConfig. We should expose the additional bootstrap options.

Anything else you would like to add:
As an example someone may want to specify the container runtime to use via the ----container-runtime option.

Environment:

• Cluster-api-provider-aws version: 1.0.0
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[richardcase]

/assign
/lifecycle active

[richardcase]

/triage accepted

[richardcase]

/milestone v1.2.0