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:

https://github.com/nytimes/drone-gke/blob/6ac83b93954ef35b0ff2d797b3f58e2d978aa975/main.go#L516-L524

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 https://github.com/nytimes/drone-gke/issues/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.