manager: use static resource scheduler when configured

manager: use static resource scheduler when configured

Note that recompute_online_node_usage() becomes much slower when the
'static' resource scheduler mode is used. Tested it with ~300 HA
services (minimal containers) running on my virtual test cluster.

Timings with 'basic' mode were between 0.0004 - 0.001 seconds
Timings with 'static' mode were between 0.007 - 0.012 seconds

Combined with the fact that recompute_online_node_usage() is currently
called very often this can lead to a lot of delay during recovery
situations with hundreds of services and low thousands of services
overall and with genereous estimates even run into the watchdog timer.

Ideas to remedy this is using PVE::Cluster's
get_guest_config_properties() instead of load_config() and/or
optimizing how often recompute_online_node_usage() is called.

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