webadmin: Add an alert when adding a Domain to uninitialized pool

When trying to add a Storage Domain to an uninitialized Storage Pool
there should be a warning message that the Pool is uninitialized and
that the user should first add a data domain to initialize the pool

Change-Id: I30c2b37f2cf59ef9c57ff96d1ffc4c23d452819e
Bug-Url: https://bugzilla.redhat.com/887378
Signed-off-by: Tal Nisan <tnisan@redhat.com>