Support of quotas in benchmarking context

Currently all generated benchmark tenants are created with the default Openstack quotas.
These default values generally do not allow to create more than 10 servers or 10 volumes for example.

This blueprint propose to support quotas modification for generated benchmark's tenants by extended the context object,
allowing to run large scale scenarios.

Openstack quotas are set per service and therefore can be separated into three categories:
 - Servers related quotas configured via Nova
 - volumes related quotas configured via Cinder
 - Network related quotas configured via Neutron

Benchmark quotas will be defined using the following syntax in task configuration file:

"context": {
 [...]
 "quotas": {
  "nova": {
   "instances": 50,
   "ram": -1,
   [...]
  },
  "cinder": {
   "volumes": 100,
   "snapshots": -1,
   [...]
  },
  "neutron": {
   "network": 10,
   [...]
  }
 }
}

This blueprint will be implemented in 3 steps:

1. Add support for nova and cinder quotas in the context object.

2. Add support for the neutron client in the osclients object.
Currently, Neutron client is not declared in the available openstack clients.

3. Add support for neutron quotas in the context object.