Run Tempest

**Role Variables**

.. zuul:rolevar:: devstack_base_dir
   :default: /opt/stack

   The devstack base directory.

.. zuul:rolevar:: tempest_concurrency
   :default: 0

   The number of parallel test processes.

.. zuul:rolevar:: tempest_test_regex
   :default: ''

   A regular expression used to select the tests.
   It works only when used with some specific tox environments
   ('all', 'all-plugin'.)

.. zuul:rolevar:: tox_venvlist
   :default: smoke

   The Tempest tox environment to run.

Run Tempest

**Role Variables**

.. zuul:rolevar:: devstack_base_dir
   :default: /opt/stack

   The devstack base directory.

.. zuul:rolevar:: tempest_concurrency
   :default: 0

   The number of parallel test processes.

.. zuul:rolevar:: tempest_test_regex
   :default: ''

   A regular expression used to select the tests.

   It works only when used with some specific tox environments
   ('all', 'all-plugin'.)

   Multi-line and commented regexs can be achieved by doing this:

       ::
           vars:
             tempest_test_regex: |
               (?x)    # Ignore comments and whitespaces
               # Line with only a comment.
               (tempest\.(api|scenario|thirdparty)).*$    # Run only api scenario and third party

.. zuul:rolevar:: tox_venvlist
   :default: smoke

   The Tempest tox environment to run.

Document how to write multiline regex with comments
Document how to write multiline regex with comments

It is possible to add multiline and docummented regex to the
tempest_test_regex but it might not be evident at first, although very
useful for some scenarios.

This patch documents how this should be achieved to make it easier for
people in the future.

Change-Id: Id54679debedb1667cc70004b9a28fba921e50c0b
