175 implement ytt integration

[mdaugs]

implement #175

This pull request implements ytt integration. It can be enabled by setting USE_YTT=1. A lib directory needs to exist within the root directory and is automatically included for the ytt call. Data can be provided globally by setting prefixed environment variables:

YTT_VARS_PREFIX=YTT_VAR
YTT_VAR_INVENTORY_PATH=/tmp/inventory.yml
YTT_VAR_default_host=localhost

Or by providing library data files:

YTT_LIB_DATA_DEMO=/tmp/demo_data.yml

# /tmp/demo_data.yml
message: 'hello demo'

The library demo needs to exists in the ytt context (lib/_ytt_lib/demo/values.yml)

# lib/_ytt_lib/demo/values.yml
#@ data/values
---
demo: {}

Then, the loaded data can be used:

# forms.yaml
#@ load("@ytt:data", "data")
#@ load("@ytt:library", "library")
#@ demo = library.get("demo")
---
categories:
  - name: Default
    icon: bars
roles:
  - name: admin
    groups:
      - local/admins
constants:
  data_values: #@ data.values
  demo: #@ demo.data_values()
forms:
  - name: Demo Form
    type: ansible
    inventory: #@ data.values.INVENTORY_PATH
    playbook: dummy.yml
fields:
  - name: var_message
    type: text
    label: Message
    default: Object($(demo)).message
    eval_default: true