Build: extra Publican arrangement only for in-tree brand if usable

In other words, if and only if:

configured using "--with-brand=clusterlabs"
AND NOT clusterlabs brand              -- installed in the system
AND Publican recognizing "--brand_dir"  /

If the last sub-clause is not met (under short circuiting evaluation),
"common" is used as a fallback brand (no special treatment required).

Ability to explicitly prevent Publican generated documentation even
if it would otherwise be (using "--with-brand=" to ./configure)
is retained.