Default to serial execution of hosts when running the upgrade playbook

[OlleLarsson]

Is your feature request related to a problem? Please describe. For some lays the serial execution is based on a percentage on the number of hosts. For us this is not good as we want to have one node upgraded at a time

Describe the solution you'd like An solution is to always pass -e serial=1 in the run-playbook command

Describe alternatives you've considered Not setting this automatically and let the user be responsible for remembering to set it.

Additional context

Definition of done: A serial execution of 1 if enforced when running the upgrade playbook

[Xartos]

Will discussing internally, since there might be some more problems with doing this

[OlleLarsson]

I think we did discuss this @Xartos and came to the conclusion that are no problems with the mentioned approach