Updated documentation about minimum guaranteed bandwidth support
for OVN backend. This mech driver is capable of informing the
Placement API about the bandwidth resources of each chassis and
schedule a port depending on the minimum bandwidth rules applied.
This patch also updates the document pointing to the new
``resource_requeste`` blob passed to Nova with the information of
the requested resources of each port.
Related-Bug: #1578989
Change-Id: I75401fa64e1ccd814aa485a74b38092849ff2a18
82d14eee61