fuel plugin builder fails validation for upload_file task using YAQL

[Scott Hussey]

Detailed bug description:
  Fuel Plugin Builder v4.1 wont' build a plugin with deployment_tasks.yaml that includes
  a upload_file task with a YAQL expression for content. This same construct is used in
  the standard fuel-library task.
Steps to reproduce:
  1. Install FPB v4.1
  2. Build an empty plugin as instructed in plugin dev guide: 'fpb --create fuel-plugin-test'
  3. Update deployment_tasks.yaml to include a task similar to below:
# create astute.yaml on the node
- id: fuel-plugin-test-file
  type: upload_file
  version: 2.1.0
  groups: ['master', '/.*/']
  requires: [fuel-plugin-test-deployment-puppet]
  required_for: [deploy_end]
  refresh_on: ['*']
  parameters:
    path: /etc/fuel/cluster/{CLUSTER_ID}/astute.yaml
    permissions: '0640'
    dir_permissions: '0750'
    timeout: 180
    data:
      yaql_exp: '$.toYaml()'

  4. Build plugin via 'fpb --build fuel-plugin-test --debug'
Expected results:
  Successful build of plugin because fuel-library/deployment/puppet/osnailyfacter/modular/astute/tasks.yaml uses the same construct for building astute.yaml

Actual result:

Validation failed
File 'fuel-plugin-test/deployment_tasks.yaml', {'yaql_exp': '$.toYaml()'} is not of type 'string', value path '2 -> parameters -> data'

Reproducibility:
  Consistently reproducible.

Workaround:
  None

Impact:
  Unable to build a plugin using this apparently valid task.

Description of the environment:
 Operation system: CentOS Linux release 7.2.1511 (Core)
 Versions of components: FPB 4.1, Fuel 9.0/kilo branch

Additional information:
  I did validate in the fuel-plugins/fuel_plugin_builder/validators/schemas/v4.py the upload_file schema does appear to require a string type parameter for data. The task validator in the Nailgun API in fuel-web does not validate task parameters for the different task types.