Support for Creating EKS Kubernetes v1.11 Clusters

[buzzsurfr]

Why do you want this feature?
AWS launched support for v1.11 in EKS (along with cluster upgrades). We use eksctl for workshops and want to keep our workshop up-to-date with the latest EKS version.

What feature/behavior/change do you want?

• Change references from 1.10.3 (or 1.10.11) to 1.11.5.
• Update the image to use the v1.11.5 AMI.
• Add a new command to upgrade an existing eks cluster.

[christopherhein]

Thanks @buzzsurfr I'm going to split this into two issues to make tracking easier. This one will be for adding support for 1.11.