[pyramid] Only expose 'cubicweb.bwcompat' setting for "all-in-one" configuration type
The "pyramid" instance configuration does not work with "cubiwceb.bwcompat"
mode (on purpose). Yet, having the setting exposed in development.ini file
(generate by `cubicweb-ctl create --config pyramid <cube> <instance>` command)
is misleading and we want to remove it.
Thus, we only query this setting when cubicweb configuration is "all-in-one" and
drop the setting line from templated development.ini file. If the option is
found and True for any other configuration type, we issue a user warning (and
ignore the option).