Namespace filtering in template.py

The namespace filtering (-namespace, -ns) was broken in core for
template.py, leading to changes intended for one (or more) namespace(s)
bleeding into others.

This commit makes sure that the page generator is always decorated with
genFactory.getCombinedGenerator.

Bug: 67435
Change-Id: I2336ebec356fb87e487f6d266e3415dbca77a603