Reduces max instances for MIGs in each project (#83)

Previously the max instances for MIGs were 5, 10, 15 for mlab-sandbox,
mlab-staging, mlab-oti, respectively. This PR reduces those to 2, 3, 5, also
respectively. It is good for MIGs to scale out and in, but we need to set a
more reasonable upper bound in order to prevent large traffic spikes from
causing excessive GCP egress network charges.

Additionally, add MIG dfw13 to staging. dfw09 was retired a week ago, but I had
failed to remove it from the TF configs. However, we should have a least one
MIG in each project for testing.

mlab-oti/platform-cluster.tf

@@ -13,7 +13,7 @@ module "platform-cluster" {
       disk_type        = "pd-ssd"
       machine_type     = "n2-highcpu-4"
       mig_min_replicas = 1
-      mig_max_replicas = 15
+      mig_max_replicas = 5
       network_tier     = "PREMIUM"
       tags             = ["ndt-cloud"]
       scopes           = ["cloud-platform"]

mlab-sandbox/platform-cluster.tf

@@ -13,7 +13,7 @@ module "platform-cluster" {
       disk_type        = "pd-ssd"
       machine_type     = "n2-highcpu-4"
       mig_min_replicas = 1
-      mig_max_replicas = 5
+      mig_max_replicas = 2
       network_tier     = "PREMIUM"
       tags             = ["ndt-cloud"]
       scopes           = ["cloud-platform"]

mlab-staging/platform-cluster.tf

@@ -13,13 +13,13 @@ module "platform-cluster" {
       disk_type        = "pd-ssd"
       machine_type     = "n2-highcpu-4"
       mig_min_replicas = 1
-      mig_max_replicas = 10
+      mig_max_replicas = 3
       network_tier     = "PREMIUM"
       tags             = ["ndt-cloud"]
       scopes           = ["cloud-platform"]
     },
     migs = {
-      mlab4-dfw09 = {
+      mlab4-dfw13 = {
         region = "us-south1"
       }
     },