ci: helper: Add a required_deps higher order helper/decorator

ci: helper: Add a required_deps higher order helper/decorator

Since we'll depend on GitPython for repo cloning, we need to make sure
to emit a user friendly error if the module is not installed. This
patch introduces a helper which future patches will use as a decorator.
Inspiration for this helper has been taken out of lcitool where we use
an identical helper for this purpose.

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