Default controllers and computes for VMs

We currently don't have the capacity to run multinodes on baremetals.
Change the README to add volumes to the controllers and computes by
default, so that VM multinodes work out the box.

Author: MoteHue

README.rst

@@ -115,8 +115,8 @@ Generate Terraform variables:
    multinode_vm_subnet  = "stackhpc-ipv4-vlan-subnet-v2"
    compute_count    = "2"
    controller_count = "3"
-   compute_disk_size = 0
-   controller_disk_size = 0
+   compute_disk_size = 100
+   controller_disk_size = 100
 
    ssh_public_key = "~/.ssh/changeme.pub"
    ssh_user       = "changeme"