ovs-vswitchd: Do not use system routing table with --disable-system.

ovs-vswitchd: Do not use system routing table with --disable-system.

The --disable-system option indicates that the user wants to avoid using
the host's datapath.  This is also a good indication that the user does
not want to use other host resources such as the routing table, so this
commit implements that.

This fixes a failure in the test "ptap - recirculate after packet_type
change" when the host routing table contained an entry that affected the
generated flow.  Without this patch, the commands:

led to a failure in that test.

Reported-by: Timothy Redaelli <tredaelli@redhat.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2018-March/046406.html
Tested-By: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>