Common way to define pipes variables if more than one pipe is used.

At this time, pipe variables can only be defined within the pipe block, for example:

- pipe: atlassian/aws-code-deploy:0.3.2
              variables:
                AWS_ACCESS_KEY_ID: '<string>'
                AWS_SECRET_ACCESS_KEY: '<string>'
                AWS_DEFAULT_REGION: '<string>'
                APPLICATION_NAME: '<string>'
                COMMAND: '<string>' 

This becomes a redundant approach if more one pipes are used as part of different steps using the same credentials.

Having a common way to define pipe variables would make a better alternative for configurations having multiple pipes configured.