Fix crendential type problem in webhook operations in client side
Fix crendential type problem in webhook operations in client side

Remove dict type in Webhook Model since crendential value in response body
is no longer in dict form.

Change-Id: Ia441483dd20f38b28200ff814bb0963a8548a1f2
Closes-Bug: #1485830

stackforge/python-senlinclient,openstack/python-senlinclient,openstack/python-senlinclient,stackforge/python-senlinclient