replace can_fork with get_max_workers

replace can_fork with get_max_workers

This patch implements the use of the new max_workers setting from
the datacenter.cfg.

Adding a 'get_max_worker' method to the enviornment allows us to
do that and to replace 'can_fork' with that method.

can_fork isn't needed anymore as get_max_worker may simply return 0
to signal that the respective enviornment can not fork.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>