Set BOSH_VSPHERE_STEMCELL with absolute path
Set BOSH_VSPHERE_STEMCELL with absolute path

[#153197766](https://www.pivotaltracker.com/story/show/153197766)

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