Configure systemd cgroup driver for containerd

Fixes issues #19, #12, #10.

Bump version to 0.2.6.

Bump version number in README from 0.2.4 to 0.2.5

Fix new AWS tag name in all files

Change AWS tag prefix from 'kubeadm:' to 'terraform-kubeadm:'

Fix submodule README

Add TOC to submodule README

Set default values for private_key_file and public_key_file variables

Use ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub. After considering that kops
also uses these files as default values.

Fix READMEs

Bump version specifiers in examples to "~> 0.1"

Format test