copr-hv: don't require pre-defined frontend_url in main.ini

At least not for the initial qcow2 image generator.  Later, when used on
backend to actually start a real builder - we'll have that defined
properly.

roles/copr/backend/templates/provision/copr-rpmbuild/main.ini.j2

@@ -1,5 +1,5 @@
 [main]
-frontend_url = {{ frontend_base_url }}
+frontend_url = {{ frontend_base_url or "unset" }}
 distgit_lookaside_url = {scheme}://{netloc}/repo/pkgs/%(ns1)s/%(name)s/%(filename)s/%(hashtype)s/%(hash)s/%(filename)s
 distgit_clone_url = {scheme}://{netloc}/%(module)s
 rpm_vendor_copr_name = {{rpm_vendor_copr_name}}