BUG/MINOR: config: don't inherit the default balance algorithm in frontends

Tom Limoncelli from Stack Exchange reported a minor bug : the frontend
inherits the LB parameters from the defaults sections. The impact is
that if a "balance" directive uses any L7 parameter in the defaults
sections and the frontend is in TCP mode, a warning is emitted about
their incompatibility. The warning is harmless but a valid, sane config
should never cause any warning to be reported.

This fix should be backported into 1.5 and possibly 1.4.
(cherry picked from commit 743c128580ee29c8f073b4a29771a5ce715f3721)