Until this change, the config directory was assumed
to be located at '/config'. This patch retains that
default behaviour, but enables users/devs to override
that behaviour by setting the HOMEPAGE_CONFIG_DIR
variable.
* Only environment variables starting with HOMEPAGE_VAR_ and HOMEPAGE_FILE_
are supported
* The value of env var HOMEPAGE_VAR_XXX will replace {{HOMEPAGE_VAR_XXX}}
in any config
* The value of env var HOMEPAGE_FILE_XXX must be a file path, the contents
of which will be used to replace {{HOMEPAGE_FILE_XXX}} in any config
* If a substituted value contains a variable reference it may also be
replaced, but the behavior is non-deterministic
