Fix to allow for newer versions of AWS provider

This updates allows us to use newer versions of the ECS provider and makes it so that the module doesn't update the Vault task over and over again.

unseal key now accepts a 3 element array.

unseal_key has now become unseal_keys, and accepts a list as its
parameters. The list should have three elements, each an unseal key,
which is used to unseal the vault docker container.

This should make this module useful to existing vault infrastructures