Support cascading __warning__ config. option

There are some global and test config options that
are highly recommended for site-specific customization.
For example, testing content that is difficult or
impractical to bundled in the open source releases.

Introduce a special __warning__ option that contains
a CSV list of all recommended for customization option
names (case insensitive).  This option's value does NOT
get overridden from one section to the next.  Instead,
each items are added to the list.

As part of final config processing, any config option
values which differ from the stock or default values,
are removed from the __warning__ list.  Later, during
subtest and sub-subtest initialization, a warning is issued
for any item remaining on this list.

Signed-off-by: Chris Evich <cevich@redhat.com>
Update all tests: Best effort, work by default.

Update all tests that require option customization
to work out of the box, but utilize the new (special)
__warning__ option to document recommended config.
customizations were **not** made.

Signed-off-by: Chris Evich <cevich@redhat.com>
Rename __warning__ to __example__

While a warning will be issued, it's not really the subject matter
for this option-name.  Renamed all references and use of the former
__warning__ option, to __example__ instead.  All behavior remains
unchanged.

Signed-off-by: Chris Evich <cevich@redhat.com>
run_remote_tag: Exception message fixup

Signed-off-by: Chris Evich <cevich@redhat.com>