[vbox] add CPU execution cap setting

[Vasiliy Pleshakov]

Detailed bug description:
Since Fuel switched to task based deployment it drasticaly slows down my host. I need virtualbox scripts to configure CPU execution cap setting for all slave nodes to eliminate slowdown.

Description of the environment:
 Operation system: cygwin and virtualbox under Windows 7
 Versions of components: N/A
 Reference architecture: N/A
 Network model: N/A
 Related projects installed: N/A
Additional information:

See https://www.virtualbox.org/manual/ch03.html#settings-processor for additional info about CPU execution cap setting.

I suggest to add cpu_execution_cap variable in config.sh and consume it in slave-nodes-create-and-boot.sh action.
Also I suggest to create new action change_cpu_execution_cap_slave_nodes.sh to change setting for already existing slave nodes on the fly.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/320967

[OpenStack Infra (hudson-openstack)]

Change abandoned by Andreas Jaeger (<email address hidden>) on branch: master
Review: https://review.opendev.org/320967
Reason: This repo is retired now, no further work will get merged.