utils: Make CommandPath try to find the executable

Before this patch, if the executable was not one of the proposed
paths in the parameters, it would raise straight an ENOENT OSError.
With this patch, it will use distutils find_executable before
bailing out.

Change-Id: I0bfa722778e295e23f03791179bf8c53172d34b2
Signed-off-by: Antoni Segura Puimedon <toni@midokura.com>
Reviewed-on: http://gerrit.ovirt.org/36166
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>