If no volume, don't delete volume in teardown

If exception is thrown in setupClass before volume is created,
tearDownClass needn't delete volume.

Change-Id: I91c7eff8ec147b0ce3a9ea3e6c8d906ba68412c2
Closes-Bug: 1350167
Add min_client_version decorator for CLI tests

This adds the min_client_version decorator to be used in the CLI tests.
The decorator is used skip CLI tests where the command being tested
isn't available in the version of the client we're testing against.

The cmd method is pulled out of the ClientTestBase class so it can be
used by the decorator and is renamed so it does not conflict with the
cmd argument passed to it.

To start using the decorator and show how it works, two known CLI tests
are decorated for minimum version checks on their clients.

Part of blueprint minversion-check-for-cli-tests

Change-Id: I8a0ee390048870d450772fa064d0dc2daa2b7036
Add a config option to select compute xml api

This commit adds a config option to select whether the nova v2 api
has xml enabled.

Change-Id: I5db2b782e71d99705c9f071a14ab850afbeddeb8
Remove cleanup code according to TODO comment

This patch removes the method _cleanup_ports as suggested by
the TODO comment. Review [1] has merged long time ago and
manually cleaning up ports should no longer be necessary.

[1] https://review.openstack.org/#/c/46563/

Change-Id: I7c8b547373b13133f8712b5bafab7bc072e0b610
Merge "Add min_client_version decorator for CLI tests"
Merge "Add a config option to select compute xml api"
Merge "Remove cleanup code according to TODO comment"
Merge "If no volume, don't delete volume in teardown"
Merge branch 'master' of https://github.com/openstack/tempest