gate fix: no removed tail_log in devstack, disable grenade

The devstack tail_log function has been deprecated since before Queens
and it has only printed a warning since then. It has been finally
removed from devstack in I7bd440adf2ce8283e3ad3d5d09e6b2b877e2b42e.

The sahara-grenade job is failing and it will continue to fail until
the heat devstack plugin is fixed on all stable branches
(currently broken due to the usage of an older Fedora image which
doesn't exist anymore, see Id869f83a46454897c2fe7a532eebfa2863befe5e),
so disable it.

Change-Id: I921d7bac5963c66e14a6a06b100d624924914f5a

.zuul.yaml

@@ -16,7 +16,8 @@
         - sahara-tests-tempest-v2
         - openstack-tox-cover:
             voting: false
-        - sahara-grenade
+        - sahara-grenade:
+            voting: false
         - sahara-openstack-ansible-functional
     gate:
       queue: sahara
@@ -25,7 +26,7 @@
         - sahara-tests-scenario-v2
         - sahara-tests-tempest
         - sahara-tests-tempest-v2
-        - sahara-grenade
+        # - sahara-grenade
         - sahara-openstack-ansible-functional
     experimental:
       jobs:

devstack/plugin.sh

@@ -256,7 +256,6 @@ function start_sahara {
     if [ "$SAHARA_USE_MOD_WSGI" == "True" ] ; then
         enable_apache_site sahara-api
         restart_apache_server
-        tail_log sahara-api /var/log/$APACHE_NAME/sahara-api.log
     else
         run_process sahara-api "$SAHARA_BIN_DIR/sahara-api \
             --config-file $SAHARA_CONF_FILE"