webadmin: replace login target button with an image

New/Edit storage domain:
replacing login target button with an image
(preventing truncation in various translations).

Change-Id: I0dee7a0bb923f18ec5eb6370ffea9f5ff6c78f92
Bug-Url: https://bugzilla.redhat.com/927504
Signed-off-by: Daniel Erez <derez@redhat.com>