[TEST] Wait in rolling_upgrade rest test for old cluster to have all indices fully allocated

When one of the 2 nodes in the old cluster is shut down, shards that were on that node will become unassigned and be marked to be
delay-allocated, i.e. either a node with shard data for that shard must be available or the allocation of the shards will be delayed for a minute.
In the mixed cluster the replica shard might not be allocated as the primary is already on the node with the newer version and replicas are not allowed
then to be allocated to a node of an older version of ES. Once both nodes are upgraded, the delay might still be in place, and can only be nullified if there
is shard data available on the node. If there never was a shard on that node though, it will take a minute and run into the timeout checking for green.

This commit ensures that all shards are fully-allocated before we do the rolling restart scenario

Original commit: elastic/x-pack-elasticsearch@a0d9b1b043

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/10_basic.yaml

@@ -32,3 +32,9 @@
         index: test_index
 
  - match: { hits.total: 5 }
+
+ - do:
+     cluster.health:
+       wait_for_status: green
+       wait_for_nodes: 2
+       timeout: 25s