Resetting session persisnence for a VIP.

This change allows to reset session persistence, if it
was configured for a VIP. It's possible by passing None
value for session persistence through the REST API.

Change-Id: I8690850d0589065502a8eb774d51fe40851624f8
Fixes: bug #1109101
Merge "Resetting session persisnence for a VIP."