Bug 956670 - Fix static references to small gear size

https://bugzilla.redhat.com/show_bug.cgi?id=956670

Creating a new district would default to small gear size even if
DEFAULT_GEAR_SIZE has been updated.  This updates the static references
to use the DEFAULT_GEAR_SIZE instead.

Also added a test to verify that new districts created without gear size
specified use the default_gear_size instead of small