Update sigul-install.sh to check for sigul

If sigul is already installed, we can skip the installation. This is
important due to kojipkgs being unreliable. We now have the sigul
binary baked into the base image creation, so newer images should
already have sigul on board. If they do and we still try to do this
manual installation, we could still run into connection issues with
kojipkgs.

Issue: RELENG-3115
Change-Id: I3580b92b8e4c1c72493069bf977f96e36aba6a85
Signed-off-by: Eric Ball <eball@linuxfoundation.org>