feat: Advanced Templating (#823)

1. Added `helmfile build` command to print final state
Motivation: useful for debugging purposes and some CI scenarios

Ref #780 

2. Template interpolation is now recursive (you can cross-reference release fields) like:
```yaml
templates:
  release:
    name: {{`app-{{ .Release.Namespace }}`}}
    namespace: {{`{{ .Release.Labels.ns }}`}}
    labels:
      ns: dev
```
3. Experimental: Added some boolean release fields interpolation in templates:
```yaml
templates:
  release:
    name: {{`app-{{ .Release.Namespace }}`}}
    namespace: dev
    installedTemplate: {{`{{ eq .Release.Namespace "dev" }}`}}
```

Resolves #818

4. Added more template interpolations: Labels, SetValues
5. Added template interpolation for inline Values
6. Added `helmfile list` command to print target releases in simple tabular form
7. Added release names in some `helm` output messages, e.g.: `Comparing release=%v, chart=%v`

docs/writing-helmfile.md

@@ -89,6 +89,42 @@ releases:
   <<: *default
 ```
 
+Release Templating supports the following parts of release definition:
+- basic fields: `name`, `namespace`, `chart`, `version`
+- boolean fields: `installed`, `wait`, `tillerless`, `verify` by the means of additional text
+  fields designed for templating only: `installedTemplate`, `waitTemplate`, `tillerlessTemplate`, `verifyTemplate`
+  ```yaml
+  # ...
+    installedTemplate: '{{`{{ eq .Release.Namespace "kube-system" }}`}}'
+    waitTemplate: '{{`{{ eq .Release.Labels.tag "safe" | not }}`}}'
+  # ...
+  ```
+- `set` block values:
+  ```yaml
+  # ...
+    set:
+    - name: '{{`{{ .Release.Name }}`}}'
+      values: '{{`{{ .Release.Namespace }}`}}'
+  # ...
+  ```
+- `values` and `secrets` file paths:
+  ```yaml
+  # ...
+    values:
+    - config/{{`{{ .Release.Name }}`}}/values.yaml
+    secrets:
+    - config/{{`{{ .Release.Name }}`}}/secrets.yaml
+  # ...
+  ```
+- inline `values` map:
+  ```yaml
+  # ...
+    values:
+    - image:
+        tag: `{{ .Release.Labels.tag }}`
+  # ...
+  ```
+
 See the [issue 428](https://github.com/roboll/helmfile/issues/428) for more context on how this is supposed to work.
 
 ## Layering State Files