fixing sahara-engine setup in devstack

when distributed periodics are enabled and zookeeper
as backend is provided, sahara-engine startup fails.
this change is fixing that.

documentation updated with details of how to correctly setup
coordination backend.

Closes-bug: 1597688
Change-Id: Ib1b85dcd4e2b67d1ec9b577d103c689a07b93058

devstack/plugin.sh

@@ -151,6 +151,7 @@ function configure_sahara {
         # Enable distributed periodic tasks
         iniset $SAHARA_CONF_FILE DEFAULT periodic_coordinator_backend_url\
             $SAHARA_PERIODIC_COORDINATOR_URL
+        sudo pip install tooz[zookeeper]
     fi
 
     recreate_database sahara

doc/source/userdoc/advanced.configuration.guide.rst

@@ -218,6 +218,15 @@ to use periodic tasks distribution, the following steps are required:
        [DEFAULT]
        periodic_coordinator_backend_url=kazoo://IP:PORT
 
+ * Tooz extras should be installed. When using Zookeeper as coordination
+   backend, ``kazoo`` library should be installed. It can be done with pip:
+
+   .. sourcecode:: console
+
+      pip install tooz[zookeeper]
+
+   ..
+
  * Periodic tasks can be performed in parallel. Number of threads to run
    periodic tasks on a single engine can be set with
    ``periodic_workers_number`` parameter (only 1 thread will be launched by

releasenotes/notes/zookeeper-configuration-steps-48c3d9706c86f227.yaml

@@ -0,0 +1,7 @@
+---
+prelude: >
+    Documentation about distributed periodics are
+    extended with steps about installation additional
+    libs required for correct work of coordination backend.
+    Please refer Advanced Configuration Guide for
+    details.