Support multiple --config-file values

[andrewdmay]

For some use cases where there are a lot of config files, but all the differences are in the all section, it would be useful to have the other environments defined separately in a file that can be re-used for different stacks.

Primary configuration file (config.yml):

---
Environment: all
StackName: 'MyTestStack-{{ Environment }}'
Template: template.yml
Parameters:
  Environment: '{{ Environment }}'
Capabilities:
  - CAPABILITY_IAM

Environments configuration file (environments.yml):

---
Environment: dev
Region: us-east-1
---
Environment: test
Region: us-east-1
---
Environment: prod
Region: us-east-1

Then specifying --config-file config,yml --config-file environments.yml would use the combination of the two.

Some questions about this approach:

• If the same environment is defined in both files, will it try and merge the configuration, if so, which one wins?