ci: only run integration tests if $LIBVIRT_CI_INTEGRATION=1 is set

ci: only run integration tests if $LIBVIRT_CI_INTEGRATION=1 is set

Right now the jobs have no rules so they will always be created in
a pipeline. If the user's fork has no runner configured, then the
jobs will never be able to execute and the pipeline will not finish.

Even on upstream, there might be times the runner has to be taken
offline for maint work, or unexpectedly fail. We need a quick way
to disable the integration tests if we decide we don't want to
have pipelines queued until the runner comes back online.

Both these problems can be addressed by requiring a environment
variable to be set

   LIBVIRT_CI_INTEGRATION=1

This can be done in the GitLab repo CI settings for permanent
enablement. Alternatively it can be set for individual
scheduled jobs, or using a push option

  git push -o ci.variable=LIBVIRT_CI_INTEGRATION=1

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>