Fix dbconfig-common chapter for Icehouse

The script "mysql-remote-root" isn't in the openstack-proxy-node
package anymore, but in the "openstack-deploy" package, which does
not contain dependencies, which is a way more convenient for our
users. This patch fixes the manual installation procedure regarding
this issue.

This is an unchanged backport of:
https://review.openstack.org/#/c/97153/

Change-Id: I0ed25fff203e0fd4fa8937c5c6d6a5c8a63b5fdf