blueprint database-backup-recovery

Introduce a new option, mariadb_backup, which takes a backup of all
databases hosted in MariaDB.

Backups are performed using XtraBackup, the output of which is saved to
a dedicated Docker volume on the target host (which defaults to the
first node in the MariaDB cluster).

It supports either full (the default) or incremental backups.

Change-Id: Ied224c0d19b8734aa72092aaddd530155999dbc3

We upgrade the rst convention by following Documentation Contributor
Guide[1].

[1] https://docs.openstack.org/doc-contrib-guide

Change-Id: I89e437b83b0e6a7c1bbfbf4a02a530be072eca91
Partially-Implements: blueprint optimize-the-documentation-format

Created the admin directory, remove some files from
user dir and placed into admin and contributor.

For more detail, see the doc migration spec.
http://specs.openstack.org/openstack/docs-specs/specs/pike/os-manuals-migration.html

Change-Id: I84c565d7d14a4d90270a53e14ab93a10f7ffb9b7
Partially-Implements: blueprint ka-queens-doc-restructure