Rather than relying on file concatenation followed by Go text/template we'll just
use Go text/template. This makes is easier to customize a template without having
to modify libbeat since you can just overwrite a template section (like inject your own
processors section).

This works by loading all of the templates from `libbeat/_meta/config/*.tmpl` followed
by templates from `$mybeat/_meta/config/*.tmpl`. Files loaded last take precedence
(it uses go text/template ParseGlob). Finally each template is created from
`libbeat/_meta/config/default{., reference, docker}.yml.tmpl` (which can be changed
if needed, see elastic/agent).

(cherry picked from commit c81adcc)