Install Guide: Fix PASS/DBPASS usage

X_DBPASS is the MySQL password, X_PASS the keystone service one.
Fix places that used DBPASS instead of PASS.

Cherry picked from https://review.openstack.org/#/c/53444/

Change-Id: Icf210e4d1c51b74897e9dc8fbabe53968d4b2e2c
backport: havana
Merge "Install Guide: Fix PASS/DBPASS usage" into stable/havana