Fix compilation errors seen with Centos 7
    
    The ovs commit acc5df0e3cb036524d49891fdb9ba89b609dd26a has no Python 2
    support. So we cannot compile OVN with this version of OVS on Centos 7.
    This patches  uses the - openvswitch-2.12.90 with the
    commit - 36e5d97f9b09262ccc584ccb45fb06482b0cfc46 which has Python 2 support.
    
    Signed-off-by: Numan Siddique <nusiddiq@redhat.com>