add Usage base plugin and Usage::Basic plugin

add Usage base plugin and Usage::Basic plugin

in preparation to also support static resource scheduling via another
such Usage plugin.

The interface is designed in anticipation of the Usage::Static plugin,
the Usage::Basic plugin doesn't require all parameters.

In Usage::Static, the $haenv will necessary for logging and getting
the static node stats. add_service_usage_to_node() and
score_nodes_to_start_service() take the sid, service node and the
former also the optional migration target (during a migration it's not
clear whether the config file has already been moved or not) to be
able to get the static service stats.

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