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.

Change-Id: I135fcf4eced517a5ee21998949f6ad9733500384
Backport: Icehouse
Merge "Fix dbconfig-common chapter for Icehouse"