Schema is not PostgreSQL connection parameter

Temporary fix for bug 70030.

openConnectionWithParams() got a new schema
parameter in 30a82aae9cf and therefore must
not be used to probe some available databases
when checking username/password, because
schema is not at all a PostgreSQL connection
parameter - it something similar to
"current directory" in the database. PostgreSQL
can additionally search additional schemas
for objects in necessary.

This whole openPgConnection logic needs some
deeper repair; for now however, let's quickly
fix bug 70030 which is caused by that fact
that openConnectionWithParams returns magic
"Status" object and no longer a DatabasePostgres
instance we can directly talk to.

As a result, openPgConnection( "create-db" )
was returning a Status object that had
another Status object embedded in the value
field.

This reverts partially commit 30a82aae9cf7e5536eef761637a1277b7c18017e.

Conflicts:
	includes/installer/Installer.php
	includes/installer/PostgresInstaller.php
	maintenance/install.php

Bug: 70030
Change-Id: I61d5f262e60722bc08201b40b61a298d8af42223