Show Neutron floating IPs quotas on Overview

Display the correct limits and usage when Neutron is in use and the
quotas extension is enabled. If Neutron is enabled but the quotas
extensions is not supported, assume the floating IPs quota is
unlimited (a floating IP quota is expected to exist in other places,
e.g. Security and Access panel)

Because quotas may not be configured or enabled even if the extension
is available, add an 'enable_quotas' setting.

Partial-Bug: #1109140

Change-Id: Id6345f4700f0ff45be8ce8acb69cca0d4e05e14a
Merge "Show Neutron floating IPs quotas on Overview"