In case there is a minikube executable in the $PATH already, use that
for all commands. If there is none, install_minikube() will place a
newly downloaded executable in /usr/local/bin which will be used by the
full pathname, so that commands as root without /usr/local/bin in the
$PATH will work.
Signed-off-by: Niels de Vos <ndevos@redhat.com>