Add Neutron benchmarks scenario for creating and listing subnets

This patch contains:
    - scenario NeutronNetworks.create_and_list_subnets is added
    - method base.Scenario._generate_random_name is added
      to provide a unified generation of random names.
    - a piece of refactoring for scenario
      NeutronNetworks.create_and_list_networks

blueprint benchmark-scenarios-for-neutron

Change-Id: I4ba67eb3babcf48e1842904c9e39b22a01351a8b
Merge "Add Neutron benchmarks scenario for creating and listing subnets"