Add source YAML

[olblak]

Add source "YAML".
This change needed some refactoring on the source interface in order to fetch files from a scm repository.

The source yaml can be configured as follow

source:
  kind: yaml
  spec:
    file: examples/values.yaml
    key: "github.user"
  #  SCM is optional
  #scm:
  #  github:
  #    user: "{{ .github.user }}" 
  #    email: "{{ .github.email }}" 
  #    owner: "{{ .github.owner }}" 
  #    repository: "{{ .github.repository }}" 
  #    token: "{{ requiredEnv .github.token }}" 
  #    username: "{{ .github.username }}" 
  #    branch: "{{ .github.branch }}"