webadmin: Fix Volume Dialog cluster validation issue

Error message will be cleared if the user selects different
cluster. If the user changes the cluster before the call is
returned from backend, nothing will be done with the result
of the first call.

Change-Id: I457fe781fc4c54f2e705d1c0febfc73ccb2ab2ff
Bug-Url: https://bugzilla.redhat.com/883774
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>