Delete existing vm on update

Signed-off-by: Maria Shaldibina <mshaldibina@pivotal.io>
Stop existing vm before deleting

Signed-off-by: Stev Witzel <switzel@pivotal.io>
Unmount disks from existing vm on update

Signed-off-by: Stev Witzel <switzel@pivotal.io>
Use existing release in deployment record

Signed-off-by: Stev Witzel <switzel@pivotal.io>
Add acceptance test for vm update

Signed-off-by: Stev Witzel <switzel@pivotal.io>
Forget current vm id after vm delete

Signed-off-by: Stev Witzel <switzel@pivotal.io>