Enable some volumes v2 tests by sharing codes

1. create volumes v2 tests using the existent test class.
2. create a new test class for v1, which inherits v2.
3. add variable 'special_fields' to deal with the different
   field's name and reps code between v1 and v2.

This patch includes the following volumes tests:
- test_volume_metadata.py
- test_volume_transfers.py
- test_volumes_actions.py
- test_volumes_get.py
- test_volumes_negative.py

Partially implements: blueprint cinder-v2-api-tests

Change-Id: I30b8f38d1f8132ea28266a32d2c9cb912a70e2c3
Call clear_isolated_creds in scenario tests

After commit b7144eb08175d010e1300e14f4f75d04d9c63c98, tempest.scenario.manager
doesn't clear isolated credentials anymore. This patch adds a tearDownClass
method to class OfficialClientTest so isolated credentials can be cleared.

Change-Id: Ic03e0d5026e09bff6d1d20daf2ee72bfdb106145
Closes-Bug: #1343027
Fix exception when api_extensions is set to empty

test.is_extension_enabled() is not checking if
config_dict[service] list is empty

Added a check if config_dict[service] is empty the
function will return False which means no extensions
are enabled

Change-Id: I2ee64f205c393637b5fc65897f1489292781c0be
Closes-Bug: 1342516
orchestration api test_stacks wait for deletion

Wait for delete to complete after triggering it, so we avoid
a conflict exception when the test _clear_stacks cleanup tries
to delete it again.

Change-Id: I64065ad8444d814e2cd7f162b869a2f8eedb1fad
Closes-Bug: #1344989
Add links to the field guide index

This commit takes the subsection summary in the field guid index
and adds internal links to the field guide page. Previously when
viewing the field guide docs it wasn't clear how to navigate between
all the different test class pages, this is remedied by adding a link.

Change-Id: Ica93a92c57e5d5d805a8b25d7c7e17509695152b
Add unit test section to the field guide index

The unit tests are another class of tests in tempest, but they were
excluded from the field guide index. This commit adds a section at the
bottom of the index for the unit test field guide.

Change-Id: I0f081c7a94bca46322381f8a5d28d7cadf334bbc
Merge "Add links to the field guide index"
Merge "Add unit test section to the field guide index"
Merge "Fix exception when api_extensions is set to empty"
Merge "orchestration api test_stacks wait for deletion"
Merge "Call clear_isolated_creds in scenario tests"
Merge "Enable some volumes v2 tests by sharing codes"
Merge branch 'master' of https://github.com/openstack/tempest