used resource count in tenant quotas

Add the used resource count when getting quotas information for a tenant.

Actually, there's no api call to get the information on used resources.
The goal is to return the count of used resources of a tenant.

There's already a way to show quotas of a tenant. And the payload contains these values:

* floatingip
* network
* port
* router
* security_group
* security_group_rule
* subnet

It is possible to count every resource. I propose to add these values to the quotas payload:

* used_floatingips
* used_networks
* used_ports
* used_routers
* used_security_group_rules
* used_security_groups
* used_subnets

It should then be easier to check for used resources.