webadmin: Created EntityModelWidgetWithInfo widget

Refactored some code implemented for NeutronAgentWidget into the
reusable EntityModelWidgetWithInfo widget, which comprises a label,
info icon and text box, whose layout I've been able to arrange
reasonably well.

Change-Id: I5cc54996bcd25062cb929d59b257688adf431126
Signed-off-by: Lior Vernia <lvernia@redhat.com>