Add command names to simplify tests

Signed-off-by: Phan Le <ple@pivotal.io>
Added StemcellManager and Cloud/Infrastructure

- StemcellManager handles extracting the stemcell tarball, parsing the
  stemcell manifest, uploading the stemcell to the infrastructure, and saving
  a record of the stemcell in the StemcellRepo
- stemcell.Infrastructure represents the API needed to manage stemcells in the
  cloud
- cloud.Cloud represents the composite API of the infrastructure providers

Signed-off-by: Phan Le <ple@pivotal.io>
Made SHA1 capitolization consistent

Signed-off-by: Phan Le <ple@pivotal.io>
Moved integration fixtures under the integration package

Signed-off-by: Phan Le <ple@pivotal.io>
Added stemcell tarball existence validation
Added bin/cpi execution

- STDIN/STDOUT are json payloads
- CloudFactory determines which CPI deployment job template to use
- JobInstaller now returns an InstalledJob record
- Added integration tests to verify bin/cpi inputs using a more complicated
  bin/cpi.erb fixture