engine : placeholders of child commands aren't cleared on exception

When one of the child commands fails or throws
a VdcBllException if not all child commands
are executed clearAsyncTasksWithOutVdsmId is
not being invoked.

On VdcBllException we need to invoke method
clearAsyncTasksWithOutVdsmId on all child
commands to clear the place holder of commands
that have not been executed.

Change-Id: I457a1f9357fda9232a8757984d6936d9c977aa62
Bug-Url: https://bugzilla.redhat.com/1126204
Signed-off-by: Ravi Nori <rnori@redhat.com>