* Decrease the image size by not installing unnecessary packages.
* Move the option 'backup_docker_image' to separate database config
section.
Change-Id: I3362e1f8a7dc21b89c060352f2db2f138c16db07
Significant changes:
* Using docker image to install datastore.
* Datastore image is common to different datastores.
* Using backup docker image to do backup and restore.
* Support MariaDB replication
* Set most of the functional jobs as non-voting as nested
virtualization is not supported in CI.
Change-Id: Ia9c97a63a961eebc336b70d28dc77638144c1834