tests/centosci: Increase disk size of minikube VMs

With more recent versions of Kubernetes, cluster setup seems to
consume more space leading to the edge of required percentage of
available space(default 30%) on the file system used by monitor
database. Therefore increase the disk capacity to 15G.

Status:
  Ceph:
    Capacity:
      Bytes Available:  63043710976
      Bytes Total:      64424509440
      Bytes Used:       1380798464
      Last Updated:     2023-06-19T12:00:15Z
    Details:
      MON_DISK_LOW:
        Message:   mon a is low on available space
        Severity:  HEALTH_WARN
    Fsid:          e91d9c48-129a-4ced-b626-402901563c69
    Health:        HEALTH_WARN
    Last Checked:  2023-06-19T12:00:15Z

Signed-off-by: Anoop C S <[email protected]>

Author: anoopcs9

tests/centosci/sink-common.sh

@@ -21,7 +21,7 @@ NODE_COUNT=${NODE_COUNT:-"3"}
 MEMORY=${MEMORY:-"4096"}
 CPUS=${CPUS:-"2"}
 NUM_DISKS=${NUM_DISKS:-"2"}
-DISK_SIZE=${DISK_SIZE:-"10g"}
+DISK_SIZE=${DISK_SIZE:-"15g"}
 DISK_CONFIG=${DISK_CONFIG:-" --extra-disks=${NUM_DISKS} --disk-size=${DISK_SIZE}"}
 
 image_pull() {