Add ability to override use_dynamic_credentials

Currently there is no ability to override use_dynamic_credentials from
pillar, however in case when Barbican should be tested we need to have
static credentials to launch full tempest. We could set
use_dynamic_credentials dynamically on inside runtest modules, but it is
not desired automation, because Barbican tests can be successfuly 
launched with dynamic credentials set to true as well.

Change-Id: I56afa934e201e98aa94bd3df7e5937435d26e33e
diff --git a/metadata/service/tempest/init.yml b/metadata/service/tempest/init.yml
index 9613452..0faa34e 100644
--- a/metadata/service/tempest/init.yml
+++ b/metadata/service/tempest/init.yml
@@ -27,6 +27,7 @@
     runtest_tempest_log_file: /home/rally/test/tempest.log
     runtest_tempest_regenerate_conf: false
     runtest_tempest_public_net: public
+    runtest_tempest_use_dynamic_credentials: true
   runtest:
     enabled: true
     openstack_version: stable/${_param:openstack_version}
@@ -46,6 +47,8 @@
           public_network_id: ${_param:runtest_tempest_public_net}
       cfg_dir: ${_param:runtest_tempest_cfg_dir}
       cfg_name: ${_param:runtest_tempest_cfg_name}
+      auth:
+        use_dynamic_credentials: ${_param:runtest_tempest_use_dynamic_credentials}
       DEFAULT:
         log_file: ${_param:runtest_tempest_log_file}
       compute: