Error Conflict: Multiple possible networks found, use a Network ID to be more specific. (HTTP 409)

[Jacques Malaprade]

OS: Ubuntu-16.04
Rally version: 0.10.1~dev31

When running the task `rally task start boot-server-and-attach-interface.
yaml` with the following yaml file:

```
  NovaServers.boot_server_and_attach_interface:
    -
      args:
        flavor:
            name: "compute-A"
        image:
            name: "Ubuntu-16.04"
        network_create_args: {}
        subnet_create_args: {}
        subnet_cidr_start: {}
        boot_server_args: {}
      runner:
        type: "constant"
        times: 1
        concurrency: 1
      context:
        network: {}
        users:
          tenants: 1
          users_per_tenant: 1
        quotas:
          neutron:
            network: -1
            subnet: -1
      sla:
        failure_rate:
          max: 0
```

The following error occurs:

` ITER: 1 END: Error Conflict: Multiple possible networks found, use a Network ID to be more specific. (HTTP 409)`

From previous bug reports concerning the same error message but with different plugins I tried to provide the "nics:" key to the servers arguments key such as:

```
args:
        flavor:
            name: "compute-A"
        image:
            name: "Ubuntu-16.04"
        network_create_args: {}
        subnet_create_args: {}
        subnet_cidr_start: {}
        boot_server_args: {}
        nics: "6ee0b526-f33c-4717-800a-ec257cbdc839"
```
with the following error:

`ITER: 1 END: Error TypeError: run() got an unexpected keyword argument 'nics'`

I then tried to add it to the "context:" key such as described here (https://bugs.launchpad.net/rally/+bug/1695245) and here (https://bugs.launchpad.net/rally/+bug/1592292):
```
      context:
        network: {}
        users:
          tenants: 1
          users_per_tenant: 2
        quotas:
          neutron:
            network: -1
            subnet: -1
        nics: "6ee0b526-f33c-4717-800a-ec257cbdc839"
```

with the error:

`There is no Context plugin with name: 'nics'`

I also tried to add key:value pairs to various other locations such as:
```
       network_create_args:
            network: "6ee0b526-f33c-4717-800a-ec257cbdc839"
```
with error:
`Multiple possible networks found, use a Network ID to be more specific. (HTTP 409)`

or with:
```
       network_create_args:
            net-id: "6ee0b526-f33c-4717-800a-ec257cbdc839"
```
with error:
`Multiple possible networks found, use a Network ID to be more specific. (HTTP 409)`

or with:
```
      context:$
        existing_network: "6ee0b526-f33c-4717-800a-ec257cbdc839"$
```
with error:
```
Reason(s):
 '6ee0b526-f33c-4717-800a-ec257cbdc839' is not of type 'object'
```

Many alternatives have been tried but it is proving impossible to assign a network in this plugin which would then boot an instance and attach the interface.

[Andrew Price (andrpric)]

You can specify the net-id like:

  NovaServers.boot_server_and_attach_interface:
    -
      args:
        flavor:
            name: "compute-A"
        image:
            name: "Ubuntu-16.04"
        auto_assign_nic: true
        nics:
          - net-id: bebcebae-06a3-4796-8713-4375b02ae0fb
      runner:
        type: "constant"
        times: 1
        concurrency: 1
      context:
        network: {}
        users:
          tenants: 1
          users_per_tenant: 1
        quotas:
          neutron:
            network: -1
            subnet: -1
      sla:
        failure_rate:
          max: 0