webadmin: Add a warning when detaching a Storage Domain

Add a warning indicating that the entities reside on the Storage Domain
will be removed from the setup.

Change-Id: Id18794fe37e9c3717fa37abac5bf69a928c800c9
Bug-Url: https://bugzilla.redhat.com/1138133
Bug-Url: https://bugzilla.redhat.com/1138119
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>