Args templating engine

[sahil-lakhwani]

Signed-off-by: sahil-lakhwani sahilakhwani@gmail.com

Adds a template rendering engine to allow templated args for helm apps.
The args passed to helm app would be allowed to use all the template functions from the standard Go template library and the Sprig library.

There are some functions added from the Helm library called from here, and other two called "include" and "lookup"

Note that the values passed to the template itself have to be a constant, meaning - the args can't refer to any values from values.yaml

Example:

apiVersion: v1
kind: kbrew
app:
  args:
    rbac.create: '{{ if (lookup "v1" "ServiceAccount" "" "admin") }}true{{ else }}false{{ end }}'
  repository:    
    name: banzaicloud-stable
    url: https://charts.bitnami.com/bitnami
    type: helm
  name: bitnami/mysql
  namespace: default
  version: 8.5.4

The value for rbac.create is resolved to true only if there's a ServiceAccount named admin present