toaster: Allow toaster to start without pytz

This patch allows toaster to start without pytz.

Django can work with or without pytz, but in the
time zone fix I mistakenly added a hard dependency
on this module.

This patch eliminates the hard dependency.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>