Adds env var support for already uploaded stemcell.
Adds env var support for already uploaded stemcell.

- Reduces the time to upload a stemcell in slower testbeds.

[#151933935](https://www.pivotaltracker.com/story/show/151933935)

Signed-off-by: Tushar Aggarwal <85b416bef673802da3543229cca08f19614abd97@vmware.com>

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