ansible-scylla-node: Stop starting seeds in parallel

Ever since raft was introduced, starting seeds in parallel sometimes
results in errors due to the other seed being in an unknown gossip
status when the other tries to start.

ansible-scylla-node/tasks/common.yml

@@ -326,20 +326,11 @@
       include_tasks: generate_tokens.yml
       when: token_distributor is defined
 
-    # Start all seeders together since we don't know which one is the "first" - so let them figure it by themselves
-    - name: start scylla on seeders
-      service:
-        name: scylla-server
-        state: started
-      become: true
-      when: broadcast_address in scylla_seeds or inventory_hostname in scylla_seeds
-
-    - name: Wait for CQL port on seeders
-      wait_for:
-        port: 9042
-        host: "{{ listen_address }}"
-        timeout: "{{ scylla_bootstrap_wait_time_sec }}"
-      when: broadcast_address in scylla_seeds or inventory_hostname in scylla_seeds
+    - name: Start seeders serially
+      run_once: true
+      include_tasks: start_one_node.yml
+      loop: "{{ groups['scylla'] }}"
+      when: hostvars[item]['broadcast_address'] in scylla_seeds or item in scylla_seeds
 
     - name: Start scylla non-seeds nodes serially
       run_once: true