DONT MERGE: launching-containers-fleet: add global units

@jonboulle  would love your thoughts on this. Will merge after global units ship in an alpha image.
s/pieces of higher-level systems/components of higher-level orchestration systems/ IMO
s/Openstack/OpenStack/

I wouldn't say "Since submitting a global unit...". Instead just "There are two `fleetctl` commands to view units in the cluster: `list-unit-files`, which shows the units that fleet knows about and whether or not they are global, and `list-units`, which shows the current state of units actively loaded into machines in the cluster"