[pve-devel] Add 'max_workers' setting to datacenter.cfg and use it

[Thomas Lamprecht]

The new settings 'max_workers' describes how many workers should be started
on a 'mass action' like 'stopall' 'migrateall' or the actions from the ha-manager.

This adds the option in (1 cluster)
Implements it for 'migrateall' and 'stopall' (2 manager)
and also for the ha-manager (3 ha-manager)

If other places are known were the option would make sense feel free to implement it :)

cheers,
Thomas