webadmin: NewEditStorageModelBehavior refactor

- Refactor items availability logic in NewEditStorageModelBehavior
  (I.e. extract the numerous conditions into boolean flags).
- Remove the deprecated export Iscsi/FCP models from the
  items list (simplifies the availability conditions).

Change-Id: Ib8e1914811367726d46c1bfaea08aba37174c11e
Signed-off-by: Daniel Erez <derez@redhat.com>