Consider allocating more vCPUs to iguana Jenkins isotesters

The machine has 12 physical cores, so 24 (hyperthreaded, virtual) cores.

Apparently we're only allocating 17 of these 24 cores to guests:

isobuilder5.xml:  <vcpu placement='auto'>4</vcpu>
isobuilder6.xml:  <vcpu placement='auto'>4</vcpu>
isotester6.xml:  <vcpu placement='auto'>3</vcpu>
isotester7.xml:  <vcpu placement='auto'>3</vcpu>
isotester8.xml:  <vcpu placement='auto'>3</vcpu>

So it might be that we're under-using the hardware. I did not check Munin though.

If this is not intended, I plan to:

• allocate as many vCPUs as I think is reasonable to the isotesters
• accordingly increase the number of CPU cores given to the L2 "TailsToaster" VM they run
  • Note: I did this on ant01 already: modules/tails_secrets_jenkins/files/jenkins/slaves/worker1.ant01/TailsToaster_config/vcpus.yml
• benchmark