Add context as param to cleanup function

In nova-network, when we start
to cleanup the stuffs it did when handle exception
but the function is following in objects/fixed_ips.py
def disassociate(self, context):
so the context param is lost and it can lead to problem.

Change-Id: I66a1681f47fc69bf0fd92137ee27481659833bef
Closes-Bug: #1350268
Merge "Add context as param to cleanup function"