This variable can be now set in Devstack's config file and in
such case Devstack will not set it automatically to value most
likely correct for the distro.
By default this value is empty string and in such case Devstack
will work in exactly same way as it was before this patch and
will determine automatically what name should be used there.
In addition in case of Ubuntu package $MYSQL_SERVICE_NAME-server
will be now installed instead of mysql-server always.
This will allow to easy configure e.g. CI job which will run using
Mariadb instead of Mysql on Ubuntu.
Change-Id: I25af0b54ad235b08c6c399b4125c737acf57ee2e
d54a1c6869