[Enhancement] include subdirectories for auto-deploy manifests

[iwilltry42]

What

This PR makes the deploy controller scan the manifests directory recursively, i.e. also yaml files living in subdirectories will be deployed upon k3s start.

Why

In k3d, we can mount volumes into the k3s server container. To get custom manifests into the auto-deploy folder inside the k3s container, we have three options:

1. we mount every file separately -> this should work
2. we mount a whole directory into this path -> manifests created by k3s may overwrite what we put there and also write back to the host system (ro won't work, since k3s will fail to write and error out)
3. we mount a whole directory into a subdirectory -> this works around the problems of (1) and (2), but only works, if the deploy controller also scans subdirectories

Fix: k3d-io/k3d#122

[erikwilson]

Thanks @iwilltry42!