pass disk identifier as string to agent in settings.json
pass disk identifier as string to agent in settings.json

 * This makes disk identifiers a consistent type with AWS and OpenStack
 * This also allows for consistent json unmarshalling in the Go agent

Signed-off-by: Michael Maximilien <202c6131ee8b1472f564bb062d6f9213961ca3fd@us.ibm.com>

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