Add root_helper to quantum agents.

Add root_helper to quantum agents.

When running commands that require root privileges, the linuxbridge,
openvswitch, and ryu agent now prepend the commands with the value of
the root_helper config variable. This is set to "sudo" in the plugins'
.ini files, allowing the agent to run as a non-root user with
appropriate sudo privilidges.

If root_helper is changed to "sudo quantum-rootwrap",
then the command being run will be filtered against lists of each
agent's valid commands in quantum/rootwrap. See
http://wiki.openstack.org/Packager/Rootwrap for details.

Fixes bug 948467.

Change-Id: I549515068a4ce8ae480905ec5eaab6257445d0c3
Signed-off-by: Bob Kukura <rkukura@redhat.com>