Respect Mailman Core's default liststyle config

When creating a new list, a selection of available styles is provided. In Mailman core, it is possible to provide a default liststlye with the following option:

[styles]
# The default style to apply if nothing else was requested.  The value is the
# name of an existing style.  If no such style exists, no style will be
# applied.
default: legacy-default

When creating lots of lists via postorius, it would be helpful if this setting is respected by preselecting not the first style in the list, but the default style.