webadmin: new pool - update disks list on template change

Ensure that IsDiskAvailable event sent on template change
(needed for updating the disk list in Resource Allocation).

Change-Id: Iced3c362de531ef3c4754ef3130563c7f6f47d1f
Bug-Url: https://bugzilla.redhat.com/957044
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>