Add support for n-g-s ml2 plugin

This patch allows to configure networking-genric-switch ml2 neutron
plugin by using the following pillar:

  neutron:
    server:
      backend:
        mechanism:
          ngs:
            driver: genericswitch
      n_g_s:
        enabled: true
        coordination:
          enabled: true
          backend_url: "http:/asdassadas"
        devices:
          s1brbm:
            options:
              device_type:
                value: netmiko_ovs_linux
              ip:
                value: 198.72.124.184
              username:
                value: ngs_ovs_manager
              password:
                value: password

Related-Prod: PROD-25542 (PROD-25542)

Change-Id: Ie9a23ad4b874a3b6eb2fb3a4e0e41b2018611da6
5 files changed