Allow setting of volume backend name while creating volume type

Summary:
Currently when a user creates a new volume type through the “Create Volume Type” action in Admin -> Volumes -> Volume Types tab, there is only one input field that the user can provide a value: volume type name. But the process of setting up a new volume type can also involve setting up volume backend name associated with the volume type. To set up the volume backend name, a user creates a new Extra Specs key/value pair through the “View Extra Specs” action. This is an additional and separate process from the volume type creation process. These two processes should be combined into one clean process that allows the setting of volume backend right within the creation of a new volume type.

Motivation:
Horizon should provide a process that helps to guide users into completing a task in a more intuitive way which consumes less time.

Description:
The purpose of this blueprint is to combine the steps to set up volume backend name for a volume type into the volume type creation process.

UX:
A new field “Volume Backend Name” field will be added to the “Create Volume Type” dialog, below the volume type name field. The input field for “Volume Backend Name” will be a dropdown menu containing a list of available volume backend names for users to select from. The list of available volume backend names will be provided by a Cinder command. The dropdown menu will also include “None” as an option for the case that users are not ready to specify the volume backend name.

Outside Dependencies:
N/A (The required Cinder API is already available)

Requirements Update Required:
N/A

Doc Impact:
N/A