Make secret & access key optional to enable instance roles

[nitehawk]

By making the access key id and secrets optional, the AWS SDK is able to take advantage of instance profiles and any access that the instance has. This removes the need to explicitly track key information in the config or environment.

This should also allow the SDK to utilize the local AWS cli credential cache.

[martinfoakes]

This also fixes the issue that occurs if you're trying to setup this plugin to access the keys via env variables for CI purposes (e.g. secretAccessKey: process.env.AWS_KEY). With isRequired() in play, and the keys set up as above, you get a ValidationError: aws.secretAccessKey is a required field error at build time