Add way to helm uninstall

[applejag]

Suggest to use similar "action" field as the kubectl step type.

Eg:

my-stage:
  my-step:
	  helm:
		  action: uninstall
			name: my-release
			namespace: prod
			cluster: cluster-config

All the flags needs to be evaluated how they should fit in here. The chartVersion flag isn't compatible with uninstalling, to begin with.

Suggested hard-coded valid enum values for the action field:

• upgrade --install (default)
• upgrade
• install
• uninstall

Maybe we want to support more? Such as also:

• rollback
• test
• lint

Do we have a bunch of statements on all fields like "can only be used on action upgrade, upgrade --install, ..." or do we have a more dynamic method of declaring fields? Goes together a little with #141