Wait for OVN dbs also along with sockets
Wait for OVN dbs also along with sockets

When OVN is setup from distro packages, the
main service is ovn-central which when restarted,
restarts ovn-northd, ovn nb and db services.

And during the restart ovn dbs(ovnnb_db.db and ovnsb_db.db)
are created, which may sometime takes time as seen with
ubuntu jammy tests[1].

We already checking for socket's file to be available,
let's also check for db files as without it ovn-*ctl
operations succeed but changes are not persisted until
db files are available and changes are lost with the restart.

[1] https://review.opendev.org/c/openstack/devstack/+/839389

Change-Id: I178da7af8cba8bcc8a67174e439df7c0f2c7d4d5

nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack