engine : Add custom json deserializer for VdcActionType

If a VdcActionType is removed json deserialization
of VdcActionType fails leading to exceptions in
Commands Cache initialization.

Change-Id: Ia8236c4f57ceea557b1e60aa7c40e180c0a43b98
Bug-Url: https://bugzilla.redhat.com/1133041
Signed-off-by: Ravi Nori <rnori@redhat.com>