Add support for vmdk images.

The vmdk images are needed for testing against VMwareVCDriver. The type is auto-detected by using the ```file```. With this and with ``` compute.allow_tenant_isolation true``` I was able to get:

```
----------------------------------------------------------------------
XML: /root/tempest/nosetests.xml
Slowest 5 tests took 3140.01 secs:
    675.48    TestMinimumBasicScenario.test_minimum_basic_scenario
    640.41    TestServerBasicOps.test_server_basicops
    608.26    ServerActionsTestJSON.test_pause_unpause_server
    608.21    ServersNegativeTestJSON.test_pause_paused_server
    607.64    ServersNegativeTestXML.test_pause_paused_server
----------------------------------------------------------------------
Ran 1129 tests in 14712.407s

FAILED (SKIP=43, errors=7, failures=4)
```