Bump version to 2015.2
Bump version to 2015.2

Start Liberty cycle.

Change-Id: I3e23cc7c977690d8f2f6d0b34d31ec1c0c903a21

openstack/senlin,Alzon/senlin,Alzon/senlin,openstack/senlin,openstack/senlin,stackforge/senlin,tengqm/senlin-container,stackforge/senlin,tengqm/senlin-container