Migrate should respect locally configured credentials

[JordanForeman]

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Actual Behaviour

Regardless of the user's local environment configuration, all database migrations are run with mock access/secret key ids. This is both poorly documented and inconsistent with expecations; it also results in failed interactions with tables as noted in #183

Expected Behaviour

The existing behaviour is fine as a fallback; however, if AWS_ACCESS_KEY_ID and/or AWS_SECRET_ACCESS_KEY are set on the environment (as is common in most AWS-based infrastructures), those should be used instead.

Steps to reproduce it

1. Configure your local environment with either (or both) AWS_ACCESS_KEY_ID/AWS_SECRET_ACCESS_KEY
2. Run sls dynamodb migrate to migrate tables configured in your serverless.yml
3. Verify a failure to connect by either: 3a. Using the awscli: aws dynamodb list-tables --endpoint-url http://localhost:8000 3b: Using the AWS.DynamoDB or AWS.DynamoDB.DocumentClient SDKs

You'll notice that both 3a and 3b above result in the tables created by migration not being found.

Screenshots of the issue

serverless-dynamodb-local can find the tables that it created earlier:

...however, awscli cannot not:

Would you like to work on the issue?

https://github.com/99xt/serverless-dynamodb-local/pull/204