This commit imports the clouds.yaml parsing code from the utils module, and exposes it in a way that fits the natural Gophercloud authentication flow.

Unlike the code from utils, this new `clouds.Parse` function keeps the separation between the ProviderClient (holding the cloud coordinates and a Keystone token) and the ServiceClient (holding specific endpoint configuration).

By default, `clouds.Parse` fetches its configuration from the environment, just like the openstack client would do.

Example use:

```Go
func main() {
	ctx := context.Background()
	ao, eo, tlsConfig, err := clouds.Parse()
	if err != nil {
		panic(err)
	}

	providerClient, err := config.NewProviderClient(ctx, ao, config.WithTLSConfig(tlsConfig))
	if err != nil {
		panic(err)
	}

	networkClient, err := openstack.NewNetworkV2(providerClient, eo)
	if err != nil {
		panic(err)
	}
}
```

The `clouds.Parse` function accepts several functional options that can modify its behaviour. For example, to use a `clouds.yaml` that exists in a non-standard path:

```Go
ao, eo, tlsConfig, err := clouds.Parse(clouds.WithLocations("/my/path/clouds2.yaml"))
```

It is also possible to pass a reader directly. Note that any number of options can be passed, with each of them taking precedence of the previous if there is conflict.

```Go
const exampleClouds = `clouds:
  openstack:
    auth:
      auth_url: https://example.com:13000`

ao, eo, tlsConfig, err := clouds.Parse(
	clouds.WithCloudsYAML(strings.NewReader(exampleClouds)),
	clouds.WithIdentityEndpoint("https://example.com:13001"),
	clouds.WithCloudName("osp1"),
	clouds.WithUsername("alice"),
)
```