webadmin: DiskModel - naming convention cleanup

DiskModel:
* Renamed variable names (Java conventions).
* Reorganized variables getters/setters (and remove redundant).
* Removed redundant call to deprecated GetDiskPresetList.
  (removed GetDiskPresetList from AsyncDataProvider).

Change-Id: Iac9d635a411a0f7d54cc0e316ac831f0823df3d0
Signed-off-by: Daniel Erez <derez@redhat.com>