default to root account's time zone

if performing a course copy/import and options[:time_zone] isn't
provided, use the default_time_zone on the root account (not the sub
account). sub accounts aren't currently expected to have their own
default time zones (but the database allowed it, and the code didn't
automatically look to the root account)

refs CNVS-9312

test-plan:
 - give a root account a non-Mountain Time default time zone (e.g.
   Arizona)
 - create a sub account of that root account
 - perform a course copy across DST boundaries into that sub account
   with date shifting
 - the date shifting should follow the tz rules of the root account's
   default time zone (i.e. time of day on due dates, etc., should not
   change between the source and target)

Change-Id: I6096490b7d125263421a48c552c32ed0b215f31d
Reviewed-on: https://gerrit.instructure.com/32788
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>