service: allow running openvpn as an unprivilege user (bgo #555518)

The patch uses --user, --group and --chroot command line parameteres
to do that.

spec change for RPM:
(http://fedoraproject.org/wiki/Packaging:UsersAndGroups)
Requires(pre): shadow-utils
[...]
%pre
getent group nm-openvpn >/dev/null || groupadd -r nm-openvpn
getent passwd nm-openvpn >/dev/null || \
    useradd -r -g nm-openvpn -d / -s /sbin/nologin \
    -c "Default user for running openvpn spawned by NetworkManager" nm-openvpn
exit 0

https://bugzilla.gnome.org/show_bug.cgi?id=555518
https://bugzilla.redhat.com/show_bug.cgi?id=1195057

(cherry picked from commit 4bb9236e)