In Kernel 4.10 supports changing SR-IOV to switchdev mode.
This mode allows to create VFs represontors which can manage
the SR-IOV VFs from the hypervsior.
This patch extends the tripleo::host::sriov::number_of_vf to
<physical_network>:<number_of_vfs>:<sriov_mode>,
where sriov_mode accepts legacy or switchdev.
if sriov_mode is not specified we default to legacy.
Change-Id: I578f956f2a8c6ee29a9d1ff38ee51765bcab05c1