fixes #8925 - support plugin asset manifests beneath app root
This supports Debian packaging where storing the assets manifest.yml beneath the gem installation directory isn't possible, as bundler manages gem installations.
Instead, support loading ${app.root}/public/assets/${plugin}/manifest.yml so plugin manifests can be packaged under the main Foreman root.
Related issues
Feature #8925: Load plugin asset manifests from Foreman root directory
fixes #8925 - support plugin asset manifests beneath app root
This supports Debian packaging where storing the assets manifest.yml beneath
the gem installation directory isn't possible, as bundler manages gem
installations.
Instead, support loading ${app.root}/public/assets/${plugin}/manifest.yml so
plugin manifests can be packaged under the main Foreman root.