core: Set default timezone to a Templates and VMs

When adding a new template we don't set the default timezone which cause
the UI to display a blank timezone.

This patch also address the importing VM/Template and adding an upgrade
script to set the default timezone to an already created VMs/Templates
if its timezone is null.

Change-Id: Id55b02a10c2faa17a7a50b64516d902c3d97e06d
Bug-Url: https://bugzilla.redhat.com/1087917
Signed-off-by: Shahar Havivi <shaharh@redhat.com>