core: Remove storage type parameter in create storage pool VDS command

As a part of the effort to remove the type of storage pool, the storage
type parameter in CreateStoragePoolVDSCommand was removed as it's
ignored anyway in VDSM so no need to set it. The method signature in
the broker and VDSM will remain the same for backward compatibility purposes

Change-Id: Ia7efe3008fcdcbde4336ad026249b290b6b3eed5
Relates-To: https://bugzilla.redhat.com/1038053
Signed-off-by: Tal Nisan <tnisan@redhat.com>