Code dedup in test_libvirt_volume

- externed common assertions to methods
- externed common parts to setUp():
  - test disk info
  - test connector
  - test volume name, location, iqn, data, uuid
  - test user
  - test fake connection

bp nova-tests-code-duplication

Change-Id: I83bfd8b597121c9de091283845b841680e150e76
Merge "Code dedup in test_libvirt_volume"