Add DeleteStemcell to cloud

Signed-off-by: Christian Williams <cwilliams@pivotal.io>
Add ClearCurrent to stemcell repo.

Rename vm repo's DeleteCurrent to ClearCurrent.

Signed-off-by: Christian Williams <cwilliams@pivotal.io>
Use vmRepo instead of configService

in vmManager

Signed-off-by: Christian Williams <cwilliams@pivotal.io>
Delete unused stemcells

* Move stemcellManager to deployer
* vm.Delete clears current stemcell
* Update current stemcell after vm is created