Make test_regex simpler for tempest

Earlier we are using complex regex '\[.*\b{{ test_regex }}\b.*\]'
to run test in validate-tempest role which appears to be confusing
and it does not excepts other tests. So i have modified it to make
more simpler by keep '(test_regex)' in that. ostestr will easy handle
it.

Change-Id: I0287ee7a19fca0298d9b934adde1a3112615cef3

roles/validate-tempest/README.md

@@ -17,7 +17,7 @@ Role Variables
 * `tempest_format`: venv/packages - Which tempest installation to use - either install python virtual environment
                     with installed there python modules from requirements file, or to use installed with RDO RPM packages
 * `tempest_log_file` - name of log file for tempest run
-* `test_regex` - tests regular expression for testr run, i.e. smoke
+* `test_regex` - tests regular expression for testr run, i.e. smoke or tempest.api.object_storage|keystone_tempest_plugin.
 * `run_tempest`: false/true - to run tempest or not
 * `skip_file_src`: path to skip tests file, set it empty if running without skip list: `skip_file_src=''`
 * `tempest_workers`: int - how many parallel workers to run (default is number of cores)

roles/validate-tempest/templates/run-tempest.sh.j2

@@ -26,7 +26,7 @@ ostestr -l
 ## List tempest plugins
 $TEMPESTCLI list-plugins
 
-( $OSTESTR '\[.*\b{{ test_regex }}\b.*\]' --no-pretty --subunit {% if skip_file_src != '' %}--blacklist_file={{ working_dir }}/{{ skip_file }}{% endif %}
+( $OSTESTR '({{ test_regex }})' --no-pretty --subunit {% if skip_file_src != '' %}--blacklist_file={{ working_dir }}/{{ skip_file }}{% endif %}
 {% if tempest_workers is defined %} --concurrency {{ tempest_workers }} {% endif %}
 {% if tempest_isolated|bool %} --isolated {% endif %}
 {% if tempest_tests_file is defined %} --load-list={{ tempest_tests_file }} {% endif %}