Added session_persistence property to VIP

Added new property session_persistance to the OS::Neutron::LoadBalancer
resource attribute VIP. Now user can configure session stickness feature
in Neutron LBaaS.

Change-Id: I9afd1a8c222110899a90a940e7a549278c4467b7
Closes-Bug: #1259078
Merge "Added session_persistence property to VIP"