webadmin: SanStorageModel - ensure discover proposal

iSCSI storage dialog:
ensure that 'Discover Targets' panel is expanded when
no targets are available.

Change-Id: I63a7f16020e9a5f4e8669e3441436cf5bb1f996d
Bug-Url: https://bugzilla.redhat.com/1134867
Signed-off-by: Daniel Erez <derez@redhat.com>