mariadb does not use DATABASE_USER = 'root' by default.
New versions of MySQL flavor MariaDB do not allow remote access for
'root' user by default.
This patch enables non-root users by loading DATABASE_USER variable
from stackrc (which is loaded by MYSQL_USER in localrc). It keeps 'root'
user as default for MySQL.
Change-Id: I15f5f7ca1fe91d97b3f5bf268982b133513e5e3c
6d3e6cfbfb