OS-4374: set swarm mode osdistro to fedora-atomic

Upstream name is fedora-atomic-swarm-mode to keep coe as swarm, but that means having two similar images with different osdistros. Set it to fedora-atomic in our setup as we don't care about legacy swarm anymore.

Closes #4374

magnum/drivers/swarm_fedora_atomic_v2/template_def.py

@@ -21,7 +21,7 @@ class AtomicSwarmModeTemplateDefinition(sftd.SwarmModeTemplateDefinition):
 
     provides = [
         {'server_type': 'vm',
-         'os': 'fedora-atomic-swarm-mode',
+         'os': 'fedora-atomic',
          'coe': 'swarm'},
     ]