usage: add Usage::Static plugin

usage: add Usage::Static plugin

for calculating node usage of services based upon static CPU and
memory configuration as well as scoring the nodes with that
information to decide where to start a new or recovered service.

For getting the service stats, it's necessary to also consider the
migration target (if present), becuase the configuration file might
have already moved.

It's necessary to update the cluster filesystem upon stealing the
service to be able to always read the moved config right away when
adding the usage.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>