Use vault-secrets as user-level systemd services

[rvem]

Some of our deployments use user-level systemd services which need some secrets provisioned by vault-secrets defined on system level.
However, in this case, dependencies between user-level and system-level services don't work because these services are handled by different systemd instances. Lack of such dependency may lead to weird behaviour, e.g. a service that need a secret may fail to restart after a reboot because it'll attempt to start prior to the secrets service finish.

The goal of this issue is to check if vault-secrets services can be used on the user-level and adjust them if the current implementation doesn't allow it to do that.