add juju-verify rules for ovn-central

[esunar]

It would be nice to have verification steps to safely remove ovn-central units.

Verification should include:

Shutdown/reboot: before shutdown/reboot it needs to verify that sb cluster and nb cluster will have a quorum without that member.

Remove: before removal of the unit, it needs to verify that the member is no longer part of sb and nb cluster and suggests to run cluster/leave to the operator otherwise.

Cluster status may be checked with:

• ovs-appctl -t /var/run/ovn/ovnnb_db.ctl cluster/status OVN_Northbound
• ovs-appctl -t /var/run/ovn/ovnsb_db.ctl cluster/status OVN_Southbound

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Imported from Launchpad using lp2gh.

• date created: 2022-02-03T11:26:05Z

• owner: peppepetra

• assignee: martin-kalcok

• the launchpad url

[Pjack]

@mkalcok . could we close this one? @rgildein mentioned this one should be resolved in.
#73

[mkalcok]

Yes. This was fixed by #73