CLOUDSTACK-804
CLOUDSTACK-4064: Missed renaming the config for KVM snapshots

KVM.snapshot.enabled is lowercased by f025db95 to keep the configs
uniformly lower-case. But it missed the upgrade script and the
references in SnapshotManagerImpl. This commit will fix the issue in all
locations

Signed-off-by: Prasanna Santhanam <tsp@apache.org>