Fix T657: add --sections argument to allow partial configuration.

Use the split argument list (slice) just for validation purposes
as it's substantially easier to do .contains in a string instead
of a slice. As such, pass the configSections arguments to
Configure() and check the existence of each one before showing
the options to the user.

An empty argument list is replaced by "server db app" so everything
is there negating the need to check anything else in Configure().
In the same vein the default is "server db app".

The parsing is done in app.go alongside the other flags instead
of main.go as described in T657.