Move reusable non-chef stuff out of recipes

[martinb3]

We have a fair amount of secret-management recipes in this stack. We should put it in a helper library, not a recipe included all over the place.

[martinb3]

There's a lot of functionality in recipes like elkstack::kibana_ssl that aren't really something that this cookbook wants to specialize in. I'd like to see us move those types of things we don't "specialize in" (E, L, or K) and use a library cookbook (e.g. certificate) rather than implement it ourselves.

And even if we have to keep it in elkstack, we should move it to a ruby library file under libraries/ or use an existing ruby library from a gem.