bootstrap-aws-installer: Drop "credential_source" in .aws/config.
bootstrap-aws-installer: Drop "credential_source" in .aws/config.

Setting `credential_source` is used when assuming role credentials --
that is, when running as one role, use the AssumeRole right to become
someone else.

The AWS command-line tools only do this if `role_arn`, the role to
assume, is also set -- if it is not set, it transparently falls
through to IAM role attached to the EC2 instance profile.  However,
with the `aws-sdk-go` package, used by Teleport, this configuration
produces an error.

Remove the `credential_source = Ec2InstanceMetadata` line, which isn't
necessary for the AWS CLI, and interferes with Teleport operation.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip