[micro] allow micro deploy --update where only disk_cid exists

If microbosh deployment gets into bad state and only the persistent
disk (disk_cid) remains, still allow "bosh micro deploy --update/--update-if-exists"
to continue successfully.
Merge pull request cloudfoundry/bosh#466 into develop
formatting

Signed-off-by: Yulia Tolskaya <ytolskaya@pivotallabs.com>
Properly test micro deployer update behavior for #464

Signed-off-by: Yulia Tolskaya <ytolskaya@pivotallabs.com>