rejigged mysql install procedure

Former-commit-id: d24c5107

roles/mysql/vars/Ubuntu_18.yml

 server_packages:
  - python
  - python-dev
- - libmysqlclient-dev
+ - libmariadb-dev
  - python-pip
  - libapache2-mod-wsgi
- - python-mysql.connector
- - mysql-server
- - python-mysqldb
+ - python-mariadb.connector
+ - mariadb-server
+ - python-mariadb
  - mariadb-server
 
 client_packages: