erp5: Increase parameter validation.
erp5.erp5: Do not escape publish_dict's values.

They contain section references, not immediate values, so they are intended
to be evaluated by buildout and hence must not be escaped.
erp5.erp5: Move balancer parameters to their own sub-mapping.

Balancer parameters were and are not supported, so this compatibility
breakage is not important.
The goal of this change is to make balancer parameter layout consistent
with other software-types.
Also, balancer's base TCP port is not taken from empty-string parameter
anymore.
erp5.TODO: Parameter review not a blocker anymore.

I reviewed parameters (both in code and in documentation) and think they are
now consistent and sane enough to not block a merge.