Implement schema validation for user managed configuration

[djelusic]

Config files that can be modified by users should be validated against a predefined schema. This currently applies to config/environment.yml

[djelusic]

I implemented a first version of this using jsonschema, specifically these two libraries:

https://github.com/alecthomas/jsonschema for generating the json schema from a Go struct, and https://github.com/qri-io/jsonschema for validating the config against the generated schema.

The config/environment.yml file is now validated every time it is loaded. If there are validation errors they will be shown to the user. For example, the following config file:

project:
  memory_size: string
  env:
    KEY: 12345
    KEY2: project
    KEY3: project
  stages: 
    - name: dev
      memory_size: string
      extra_field: oops
      timeout: 120
      env:
        KEY2: stage
        KEY3: stage
      functions:
      - name: ping
        env:
          KEY3: function

will produce the following error message:

Failed to validate environment.yml:
/project/stages/0/memory_size: type should be integer, got string
/project/stages/0: additional properties are not allowed
/project/memory_size: type should be integer, got string
/project/env/KEY: type should be string, got integer