core: missing cmdContext assignment in constructor

Following Change-Id: I310f5f77fff78b3232ee77fe63791425fd521516
RemoveSnapshotSingleDiskCommandBase constructor should pass
'cmdContext' to the parent class.

The missing context prevents the snapshot removal task from
being set as finished (see related BZ).

Change-Id: I5ce97a22fe994b5278104cc6f436248a0c66f140
Bug-Url: https://bugzilla.redhat.com/1119706
Signed-off-by: Daniel Erez <derez@redhat.com>