Introduces DAG-aware installation/deletion ordering to Helmfile.
`needs` controls the order of the installation/deletion of the release:
```yaml
relesaes:
- name: somerelease
needs:
- [TILLER_NAMESPACE/][NAMESPACE/]anotherelease
```
All the releases listed under `needs` are installed before(or deleted after) the release itself.
For the following example, `helmfile [sync|apply]` installs releases in this order:
1. logging
2. servicemesh
3. myapp1 and myapp2
```yaml
- name: myapp1
chart: charts/myapp
needs:
- servicemesh
- logging
- name: myapp2
chart: charts/myapp
needs:
- servicemesh
- logging
- name: servicemesh
chart: charts/istio
needs:
- logging
- name: logging
chart: charts/fluentd
```
Note that all the releases in a same group is installed concurrently. That is, myapp1 and myapp2 are installed concurrently.
On `helmdile [delete|destroy]`, deleations happen in the reverse order.
That is, `myapp1` and `myapp2` are deleted first, then `servicemesh`, and finally `logging`.
Resolves#715
7666e95690