pkg: Fix package dependencies for bhyve/libvirt

tmux is not needed since we use libvirt and not the custom bhyve compute
driver.
nova: Add option for disabling usage of cow images

Currently bhyve can not boot from cow image, but only from raw one.
This allows for disabling cow images by setting USE_COW_IMAGES to False
in local.conf (localrc).
nova: Fix nova launch command on FreeBSD host

This is a workaround. There is no 'sg' equivalent on FreeBSD and we use
sudo instead. However, a better fix should be found for that.
functions: FreeBSD version of restart_service()

On FreeBSD platforms if you don't have given service enabled in
/etc/rc.conf, 'onerestart' has to be used instead of 'restart'. It will
also work for those that has an entry in /etc/rc.conf.
nova: Load modules required by libvirt on FreeBSD platform
nova: Allow for different location of libvirtd.conf on FreeBSD platform
localrc: Add example config file for compute node running with libvirt/bhyve

If you use it, don't forget to customize it with your IPs, etc.
nova: Remove plugin for deprecated bhyve native compute driver
stack: Add identification for FreeBSD-10
functions: FreeBSD version of add_user_to_group()
create-stack-user: Allow sudo to any group in addition to any user

This is necessary to execute nova-compute with libvirtd group using
sudo.
nova: Resolve workaround of running nova-compute as root with sudo

Previously, sudo was used to execute nova-compute as root. This allows
to run as any user that is member of group defined by $LIBVIRT_GROUP.
Merge pull request #17 from Semihalf/dev-libvirt-bhyve-support

Devstack libvirt bhyve support on FreeBSD