Populating object before parsing step function

[lucas-rudd]

Currently this plugin is setting the step function, state machine, and activity keys before the serverless.yml file has populated the variables.

This allows you to organize serverless.yml like such:

stepFunctions:
  stateMachines:
    ${file(config/env/${opt:stage}.yml):stateMachines}

but not like this

stepFunctions:
  ${file(config/env/${opt:stage}.yml):stepFunctions}

This proposed change populates the variables before determining whether or not a step function exists in the given config.

[horike37]

@lucas-rudd
Works great!
LGTM 😄