[OpenStack][Devstack] Devstack的db backend選擇

Devstack的db backend選擇，需於local.conf中宣告:

#mysql

enable_service mysql

#mariadb

enable_service mysql

enable_plugin mariadb  https://github.com/rafaelfolco/devstack-plugin-mariadb

#postgresql

enable_service postgresql 

WARNING: Use of postgresql in devstack is deprecated, and will be removed during the Pike cycle

但於OpenStack TC的某次會議中討論提到，由於目前mysql /mariadb的使用社群佔最大宗，所以postgreSQL的部分專案於開發時將不進行驗證
https://governance.openstack.org/tc/resolutions/20170613-postgresql-status.html