Move the assignment of existing_floating_ips before try block

Move the assignment of existing_floating_ips before try block

In function _process_external() in agent/l3/agent.py, the call to
iptables_manager.defer_apply() may throw an exception, making a
later call to _update_fip_statuses() use an un-initialized value.
This will throw its own UnboundLocalError, with the result being
no iptables rules will be applied.

Added tests to cover both the defer_apply() code exception
processing, as well as this new case where we might jump to
_update_fip_statuses() without having done any work on floating
IP addresses.

Change-Id: I0045effc9319772127758be4aacca02ab5c236cd
Closes-Bug: #1413111