Fix failing lifecycle test due to incorrect `existing_disks` type
Fix failing lifecycle test due to incorrect `existing_disks` type

[#117369019](https://www.pivotaltracker.com/story/show/117369019)

cloudfoundry-incubator/bosh-vsphere-cpi-release,cloudfoundry-incubator/bosh-vsphere-cpi-release,cloudfoundry-incubator/bosh-vsphere-cpi-release,cloudfoundry-incubator/bosh-vsphere-cpi-release,cloudfoundry-incubator/bosh-vsphere-cpi-release