Use (or set properly) system-id generated by openvswitch
Use (or set properly) system-id generated by openvswitch

In case when OVN_UUID isn't set by user, and it isn't stored
in /etc/openvswith/system-id.conf file, Devstack will reuse it.
If it's not, it will generate and store it in the
/etc/openvswitch/system-id.conf file so it can be set to same value
after openvswitch will be e.g. restarted.

In case when OVN_UUID is set by user, it will be also saved in
/etc/openvswitch/system-id.conf file to make it persistent when e.g
openvswitch will be restarted.

Closes-Bug: #1918656
Change-Id: I8e3b05f3ab83e204bc1ce895baec0e1ba515895b

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