rhel: Add support for "systemctl reload openvswitch"

rhel: Add support for "systemctl reload openvswitch"

The reload procedure will trigger a script that saves the flows and tlv
maps (using ovs-save) then it restarts ovsdb-server, it stops ovs-vswitchd,
it sets other_config:flow-restore-wait=true (to wait till flow restore is
finished), it starts ovs-vswitchd, it restore the backupped flows/tlv
maps and it removes other_config:flow-restore-wait=true (logic mostly ripped
from ovs-ctl).

It uses systemctl with --job-mode=ignore-dependencies to restart ovsdb-server
and stop and start ovs-vswitchd in order to avoid systemd to restart the other
components due to dependencies (as explained in rhel/README.RHEL.rst).

Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>