Add support for loading templates from remote locations

[montmanu]

Is your feature request related to a problem?

Currently, the plugin requires that resource manifest templates exist on disk:

drone-gke/main.go

Lines 516 to 524 in 6ac83b9

	_, err := os.Stat(t)
	if os.IsNotExist(err) {
	if t == c.String("kube-template") {
	return nil, fmt.Errorf("Error finding template: %s\n", err)
	}
	log("Warning: skipping optional template %s because it was not found\n", t)
	continue
	}

This complicates sharing / re-use of templates.

Describe the solution you'd like

An ideal implementation would support loading of resource manifest templates from a remote location; similar to kubectl apply [-f|-k]

Describe alternatives you've considered

Current alternatives / workarounds include:

• pre-fetching remote manifest templates at build time (e.g., via curl, git submodules, etc)
• use of kubectl / kustomize native features (instead of plugin)

Additional context

Any implementation of this feature request should consider potential interactions with #105 (i.e., how to load multiple templates from a remote location .. especially in the case of supporting directories as a valid argument)

[tonglil]

I think this should be done in from a separate step.
That step can download any templates and concatenate them together for templating and application.