remove session parameter from fixed_ip_get

Remove session parameter from public method fixed_ip_get.
This requires some changes to floating_ip_disassociate,
but doesn't appear to affect anything else.

Change-Id: I435bfc4f2f4175049fd164fea37b4724871e1eb1
(cherry picked from commit 88ac5d532dfdbeeed5a5c7e295e3a5962a792e41)
Eliminate race conditions in floating association

This makes associating and disassociating floating ips atomic and
idempotent. This means multiple concurrent messages will not leave
behind iptables rules and concurrent request will not cause odd
failures.

Fixes bug 1092762 and bug 1092761.

Change-Id: Idbcad6c1d2a3d4881cf7180b848ed3844fac4054
(cherry picked from commit 881a93473c32a7c7e23a8e6dcede8394053408c6)
Merge "remove session parameter from fixed_ip_get" into stable/folsom
Merge "Eliminate race conditions in floating association" into stable/folsom