Server IP resource

The server IP resource can only list.  The Resource.list method
needed to be overridden because:
* The url path contains the server_id.
* The output is a dict of networks rather than an array.

Also in this change:
* ServerIP objects have sufficient information to stand on their
  own because they contain the server_id and network_label.
* An ips(session) convenience method has been added to the Server.
* The list example has been updated to pass data to path_args so
  the list method and format the url.

The path_args parameter to list may be convenient for other
classes.  I didn't add it to the Resource.list method because
that isn't required for this change.

Change-Id: Iac77aba144d2e5b4ca4cdbf97e534d83519066ef
Merge "Server IP resource"