Added the ability to disable Mellanox SDN sync by using param `MlnxSDNSyncEnabled`
from file `neutron-ml2-mlnx-sdn.yaml`
Change-Id: Id93a8637980276298c2b7d3f6d1e7c30f1c413be
Having NeutronMechanismDrivers as a string causing issues
specially with non-ovn deployments and if the value contains a
sub-string(ovn) like sriovnicswitch, so just moving it to be a list of
strings
Change-Id: Id01951bca724417db91225efeaaeb1b85e06eb70
Related-Bug: #1836011