Add Manage/Unmanage Volume feature

Summary:
Add manage/unmanage options to "Admin -> Volumes" panel so that an admin can access existing volumes not currently managed by OpenStack.

Motivation:
This functionality exists via the Cinder CLI, so we should expose it in Horizon.

$ cinder help manage
usage: cinder manage [--id-type <id-type>] [--name <name>]
                     [--description <description>]
                     [--volume-type <volume-type>]
                     [--availability-zone <availability-zone>]
                     [--metadata [<key=value> [<key=value> ...]]] [--bootable]
                     <host> <identifier>

Manage an existing volume.

Positional arguments:
  <host> Cinder host on which the existing volume resides;
                        takes the form: host@backend-name#pool
  <identifier> Name or other Identifier for existing volume

Optional arguments:
  --id-type <id-type> Type of backend device identifier provided, typically
                        source-name or source-id (Default=source-name)
  --name <name> Volume name (Default=None)
  --description <description>
                        Volume description (Default=None)
  --volume-type <volume-type>
                        Volume type (Default=None)
  --availability-zone <availability-zone>
                        Availability zone for volume (Default=None)
  --metadata [<key=value> [<key=value> ...]]
                        Metadata key=value pairs (Default=None)
  --bootable Specifies that the newly created volume should be
                        marked as bootable

$ cinder help unmanage
usage: cinder unmanage <volume>

Stop managing a volume.

Positional arguments:
  <volume> Name or ID of the volume to unmanage.

UX:
Need to add a new "Manage Volume" action button to the "Admin -> Volumes" table. This will bring up a modal dialog allowing the user to enter volume and cinder host information.

Need to add a new "Unmanage Volume" action button to each row of the "Admin -> Volumes" table. This will simply send the volume and host name to the cinder client.

Outside Dependencies:
N/A

Requirements Update Required:
N/A

Doc Impact:
N/A