Adds volume_used to trove show <instance>

Reasons:
- There are two attributes coming from trove related to volume
  attached, total size of volume and used space of volume.
- Used space is an important attribute for a user, it would help to
  plan the resize of volume before the attached volume gets finished.

Changes:
- Adds display of used volume attribut, with "trove show" command.
- Avoids display in dicts-form.
- do_show method now displays total space, and used space for the
  atttached volume, as

    |       status      |                ACTIVE                |
    |      updated      |         2014-01-12T22:17:20          |
    |       volume      |                  1                   |
    |    volume_used    |                 0.13                 |
    +-------------------+--------------------------------------+

Change-Id: I64bddd167c1645077854bd74185886bb45fc69a0
Closes-Bug: #1265144
Merge "Adds volume_used to trove show <instance>"