[3.0] Adjust auto scaling to always use the max processes

[themsaid]

As reported in #486

If the max. number of processes is 4, while you're balancing 3 queues. The number of workers per processes needed per queue will be 1.3. When using round() Horizon will assign 1 process per queue and then it's done.

However if we use ceil() Horizon will assign 1 process per queue and then it'll assign a process for the first queue since each queue will require 2 instead of 1.

• 4 / 3 = 1.3
• round(1.3) = 1 (That's the current case)
• ceil(1.3) = 2 (That's what this PR changes)