un-leak memory in cluster settings form validation code

Previous code had unlikely code path where change of pool settings url
will cause form validation to be set up again and again etc...

In order to avoid that improbable bug I'm adding code to destroy
previous instance of form validation prior to overwriting it with new
instance.

Change-Id: I06f47903dc7e90dc66fba5858e220c421ffa1b1d