Use updated trove mysql image location

Use the updated trove mysql image at:
tarballs.openstack.org/trove/images/ubuntu/mysql.qcow2
instead of the older deprecated image at:
tarballs.openstack.org/trove/images/ubuntu_mysql.qcow2/ubuntu_mysql.qcow2

Change-Id: If76f25dfe0f975faf1883f0a6d993c21b26e1b29
Merge "Use updated trove mysql image location"