rhel/ifup: support vhost-user client mode

rhel/ifup: support vhost-user client mode

This adds support for ifup to configure client-mode sockets by exposing
two new variables $OVS_PORT_MODE and $OVS_PORT_PATH to the ifcfg
scripts.  When OVS_PORT_MODE is set to 'client', the OVS_PORT_PATH will
be passed as the vhost-server-path option.

No change is needed to ifdown because the OVSDPDKVhostUserPort type
already has an appropriate entry.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>