OVN will allocate new ports/IP addresses during the migration, if
any subnets do not have IPs free for allocation the migration will
fail.
We have omitted a step for change of firewall driver to
'openvswtich'.
Change-Id: I3e7d9ee5076375c04df2ef7edc87e18fd0617c7c
Closes-Bug: #1905554
(cherry picked from commit 57d609fa74)