Currently, when creating a new flavor, it need to
set pool group name for the first argument "pool".
For making clear between a pool and pool group,
so we should replace "pool" word with "pool_group"
at zaqarclient.
Depends-On: Id4c976a1fc9cc7fb9b72482b4e3a7f7f80f94492
Change-Id: I175e26009d4ba1ffda808c558a431f824fe856ab
Closes-Bug: #1504959