webadmin: clear connection on import SD rollback

In order to avoid stale connections data in the DB,
clear storage connection when an export/iso domain
doesn't exist on the specified path
(GetExistingStorageDomainList returns an empty list)

Change-Id: Ia2e70353494ce97a6c2ae0ad74868c1f502a309c
Bug-Url: https://bugzilla.redhat.com/1020812
Signed-off-by: Daniel Erez <derez@redhat.com>