mongodb installed by default to use it as a pool in tests with zaqar enabled as a part of devstack installation

[Venkata Mahesh Jonnalagadda]

I configured zaqar and its backend as redis in local.conf with devstack installation. And observed, mongodb installed by default.

When I checked zaqar's plugin.sh script, it seems mongodb configured by default as a pool in tests!
https://github.com/openstack/zaqar/blob/master/devstack/plugin.sh

Due to this seamless configuring of mongodb, I am seeing three issues -
1) If system already configured with mongodb seperately then it will overwrite or mislead existing mongodb server.
2) User is not aware of this secret installation of mongodb during devstack installation
3) Why there is no variable specified for pool in tests to use mongodb as backend

Any services (like mongodb, redis, zaqar client etc) to install as part of devstack installation for zaqar enabled then it has to be mentioned with variables (like ENABLED_SERVICES, ZAQAR_BACKEND etc) by the user only.

It is better to have a zaqar installation variable for pool db like ZAQAR_POOL_BACKEND. This way, user can specify required backend to install or else mongodb can be specified by default, if user didn't provided ZAQAR_POOL_BACKEND variable in local.conf.

Introduce code something like below in plugin.sh -

# Set up database backend as a pool in tests for zaqar
ZAQAR_POOL_BACKEND=${ZAQAR_POOL_BACKEND:-mongodb}

# Check pool backend and configure accordingly
if [ "$ZAQAR_POOL_BACKEND" = 'mongodb' ] ; then
     configure_mongodb
else
     echo "Only mongodb is supported as a pool in tests"
     configure_mongodb
fi