core: do not remove execution jobs with ongoing tasks

In I5973c a mechanism that removes execution jobs that cannot be
tracked after engine restart was added.
A check whether there are ongoing tasks which are related to the jobs
was missing, since if there are such async tasks then the operation
can be tracked and resumed.

Change-Id: I95b1066ca6fc015fd31101e7b551b4fae1d66904
Bug-Url: https://bugzilla.redhat.com/1099505
Signed-off-by: Arik Hadas <ahadas@redhat.com>