Big Switch Neutron ML2 plugin integration

Big Switch Neutron ML2 plugin integration

Add support for Big Switch Neutron ML2 plugin. Makes sure that the
package is present and sets up the [restproxy] section in ml2_conf.ini.

This also adds support for setting the ovs_use_veth option in
l3_agent.ini. There is no support for this in puppet-neutron l3 class
and it probably doesn't make sense adding it there, because this setting
isn't relevant for all l3 agent drivers, it's specific to
OVSInterfaceDriver. The ovs_use_veth option is also added to
dhcp_agent.ini.

Change-Id: I99635e25b2099dacce68154fe14693d6f06ac19f