Move host_architecture to kubespray-defaults (#5811)

The variable is defined in `kubernetes/preinstall` role and used in several roles. Since `kubernetes/preinstall` is not always included when `ansible-playbook` is run with tag selectors (see #5734 for reason), they will fail, or individual roles must copy the same fact definitions (as in #3846). Moving the definition to the always-included `kubespray-defaults` role will resolve the dependency problem.