Add facility for rendering Helm charts

This facility renders a Helm chart with the provided values and parses it into a list of Kuberenets runtime.Objects.

Example usage:

package main

import (
  "fmt"
  "log"
  "os"

  "gitlab.com/gitlab-org/gl-openshift/gitlab-operator/pkg/helm"
  "k8s.io/apimachinery/pkg/runtime"
  core "k8s.io/api/core/v1"

  _ "k8s.io/client-go/plugin/pkg/client/auth"
)

func main() {
  v := helm.EmptyValues()
  v.AddValue("certmanager-issuer.email", "me@example.com")

  t := helm.NewTemplate("path/to/chart")
  t.DisableHooks()

  warnings, err := t.Load(v)

  if warnings != nil {
    for _, warn := range warnings {
      log.Printf("WARNING: %+v", warn)
    }
  }

  if err != nil {
    log.Printf("%+v", err)
      os.Exit(1)
  }

  t.EditObjects(helm.ServiceSelector, helm.NewServiceEditor(
    func(s *core.Service) error { 
      // doStuff
      return nil
    })
  )

  for _, obj := range t.Objects() {
    // doStuff
  }
}

Closes #2 (closed)