core: Fix a bug in exporting template with force override

When exporting a template, the pre-existence of its images should only be
checked if the forceOveride flag is set to false

Change-Id: Ida6fbcfe00403d609f76055437a3d105ac8a8d98
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1069151