Fix package name when using heat-db-setup in rhel7

In rhel7 mysql-server is replaced by mariadb-server, using
the right name to install package.

Closes-Bug: #1395726
Change-Id: I02e79a6f8da5bd082568c22d3efc4c231dfbd8c7
Merge "Fix package name when using heat-db-setup in rhel7"