Add ability to customize containerd-config.toml

[anusha-ragunathan]

What would you like to be added:

Today, a default containerd-config.toml is made available via bootstrap.sh Add the ability to pass a custom container config to bootstrap.sh

A similar option is available when using docker as the CRI implementation via $DOCKER_CONFIG_JSON. Expose something like that for containerd as well.

Why is this needed:

Needed so custom containerd configurations can be passed to EKS bootstrapper.

[bryantbiggs]

relates to #790