core: always attempt to clear tasks without vdsm id

Even when command execution succeeds, for some reason it may leave task
placeholders that were added for some reason (For example - in case that
the commands ignores a specific failure in creating a task). 
The execution method should always delete those entries at the end of the
execution.

Change-Id: Ib98e90eb3e078761b9ad9d0a30b21c9db20bd7d5
Bug-Url: https://bugzilla.redhat.com/1103687
Signed-off-by: Liron Aravot <laravot@redhat.com>