Tolerate completely missing floating_ips

In environments, where L3 agents are disabled,
launch instances just fails due quota missing
of not necessary floating ips.

This is a Icehouse-only bugfix; The underlying
issue was fixed in Juno by a blueprint implementation
and a new config option.

Change-Id: I4f9da72b95fb6489ee3c99f0006ec7b73a7d7e87
Closes-bug: #1383614
Merge "Tolerate completely missing floating_ips" into stable/icehouse