engine: Enable read of current CD via REST

Added tracking of the current CD for VM (the value resulting from
ChangeDisk).

- On REST side we check for the 'current' matrix param in the GET
  request and if present we return the currentCd of the VM instead of
  the persistent isoPath.
- Added 'current_cd' to vm_dynamic table to store this value in
  VdsBrokerObjectsBuilder.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1014039
Change-Id: Ifd6c80984143b7d953b6d76b9863465aabce0917
Signed-off-by: Martin Betak <mbetak@redhat.com>