Temporarily decrease max instances for testing (#5573)

In order to inexpensively test that the new per-platform instance tags
added in a previous commit properly limit the number of instances
running, the s390x and arm64 VM flavor maximum instances are temporarily
decreased in staging. This will allow for verification that only the
total number of instances of a specific "flavor" controls whether the
maximum number of instances of that "flavor" has been reached.

Author: enkeefe00

components/multi-platform-controller/staging/host-config.yaml

@@ -59,7 +59,7 @@ data:
   dynamic.linux-arm64.aws-secret: aws-account
   dynamic.linux-arm64.ssh-secret: aws-ssh-key
   dynamic.linux-arm64.security-group-id: sg-05bc8dd0b52158567
-  dynamic.linux-arm64.max-instances: "160"
+  dynamic.linux-arm64.max-instances: "2"
   dynamic.linux-arm64.subnet-id: subnet-030738beb81d3863a
 
   dynamic.linux-mlarge-arm64.type: aws
@@ -342,7 +342,7 @@ data:
   dynamic.linux-s390x.region: "us-east-2"
   dynamic.linux-s390x.url: "https://us-east.iaas.cloud.ibm.com/v1"
   dynamic.linux-s390x.profile: "bz2-2x8"
-  dynamic.linux-s390x.max-instances: "4"
+  dynamic.linux-s390x.max-instances: "2"
   dynamic.linux-s390x.private-ip: "true"
   dynamic.linux-s390x.instance-tag: stage-s390x