Option for users to externalize provider configuration

[tkyi]

What happened:
Currently users need to provide the entire AWS provider config as part of screwdriver.yaml.
https://docs.screwdriver.cd/cluster-management/aws-integration#job-provider-configuration

However this can lead to lots of config duplication, if same account is used in multiple pipelines and any changes will need updates to multiple git repositories.

What you expected to happen:
Give users an easy way to mention a references to provider configuration in screwdriver.yaml so that users can maintain config in a central place.

jobs:
  main:
    requires: [~pr, ~commit]
    image: aws/test/image-x86_64-standard:3.0
    provider: git_organization/repo:path

Screwdriver must fetch the provider from the given git path and apply to job configuration.