Ansible ad-hoc command to reboot servers in serial with a pause
@Argyle explained the --fork 1 trick to achieve a serial run.

I suggest you have a look at the reboot module that might be better suited than running a command directly. 

A quick example that will reboot all your nodes one by one waiting for each to be fully booted before moving to the next one.

ansible -i inventory nodes --forks 1 -m reboot 


Pass needed parameters to the module depending on your requirements with the -a option.