pyrmont: temporarily switch to running altair branch

Necessary for Eth2 2 hard-fork that is going to happen on Pyrmont.
Will revert back to the other branches once changes reach them.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ansible/pyrmont.yml

@@ -11,6 +11,7 @@
 
 - name: Deploy pyrmont Metal beacon nodes
   become: true
+  serial: '{{ serial|default(4) }}'
   hosts:
     - nimbus-pyrmont-metal
   roles:
@@ -23,7 +24,8 @@
         # Using whole libp2p branch name would be far too long.
         beacon_node_service_name: 'beacon-node-{{ beacon_node_network }}-{{ node.name }}'
         beacon_node_build_frequency: '{{ node.build_frequency }}'
-        beacon_node_repo_branch: '{{ node.branch }}'
+        # Temporary until the usual branches get the Altair changes.
+        beacon_node_repo_branch: 'altair'
         beacon_node_discovery_port: '{{ 9000 + port_offset }}'
         beacon_node_listening_port: '{{ 9000 + port_offset }}'
         beacon_node_metrics_port:   '{{ 9200 + port_offset }}'