Fix a bug some optional field in LBaaS are mandatory

* VIP connection_limit
* Pool Member weight

Closes-Bug: #1241209
Closes-Bug: #1241004

The change of create_stubs in openstack_dashboard/test/helpers.py
is cherry-picked from commit fa3a039d0585c8c995504c09657070af4025305f.
It allows us to create_stubs decorator for sub methods with arguments.

Change-Id: I337215938645f6785393925f201567f2143d133e
(cherry picked from commit c9f4e27aa5c60bc997e70b4982856bbf4a23372b)
Merge "Fix a bug some optional field in LBaaS are mandatory" into stable/havana