Make 'IP address' field optional for LBaaS VIP creation

Since 'IP address' argument is optional in the CLI command, it
should not be mandatory for the field "Specify a free IP address
from the selected subnet". This patch sets 'required=False' for
this field. Some LBaaS VIP test data cleanup is also done.

Closes-Bug: #1386616

Change-Id: I112919d12ca3b4f40d9b907b7f47b64502d5ae7c
Merge "Make 'IP address' field optional for LBaaS VIP creation"