issue#308 allow multiple documents to be selected in document selector

[mszygenda]

This PR addresses following issue:
#308

Use-case:

I want to assert whether or not all manifests of type "Deployment":

• eg. define default resources section.
• eg. follow naming convention
• eg. set right namespace

I want this assertion to work for current and future deployments I might have in my chart.
I have to use document selector because I keep different types of k8s manifests in a single YAML file (eg. foo-application.yaml contains definition of Deployment as well as the Service)

USAGE:

suite: Document Selector is matching many documents
templates:
  - "templates/application-*.yaml"
tests:
  - it: deployment names should end with -deployment suffix
    documentSelector:
      path: kind
      value: Deployment
      matchMany: true
    asserts:
      - matchRegex:
          path: metadata.name
          pattern: -deployment$

It has one limitation though - every template that you select for your test-suite has to have at least one matching manifest. The test will fail otherwise.

Happy to hear any feedback you might have. The code might need some styling fixes etc. but functionally it seems to be doing what it's supposed to do.

[mszygenda]

Ah, actually - we should check if selection generates more than 1 result across all template file - to keep full backward compatibility. I guess it needs improving.

[mszygenda]

Addressed it.