rootwrap: Move bhyve related filters to /etc/nova/rootwrap.d/freebsd.
rootwrap: Remove /usr/local/{bin,sbin} from exec_dirs in rootwrap.conf.

Instead of having these directories in the file which is used in Linux
installations aslo, the devstack script will add those if host platform is
FreeBSD, which uses /usr/local/ for every executable not included in the
base system.