Restore "Add scenario 'test_preserve_preexisting_port'"

This reverts commit 8ec99feafebd5ee67a0f4c34bcd5a30d8d42ed97

The neutron job on stable/icehouse was running more tests than what the
neutron-icehouse compat job was running against tempest changes on
master, that's why we didn't catch this when the change on master to add
the test was under review.

Note that stable/juno neutron jobs were not running these tests either
because the router extension isn't enabled.

The difference was we didn't have change
I7c80e3bfe4962c3e8c94736af21ff215cd98f7f3 on stable/icehouse or
stable/juno in devstack before the new test merged, which broke
stable/icehouse.

Now that devstack is fixed on stable and will run these scenario tests
we add a 'preserve_ports' config flag to compute-feature-enabled so it
can be toggled from devstack.
For stable branches we won't run the test since the code doesn't work in
Nova on stable (and won't be backported). For master devstack, we'll set
preserve_ports=True so the test does get run. We can remove the option
after juno-eol happens.

Devstack change: I214baa3b861e29bedf6bb7b50534ac2286676dd1

Related-Bug: #1431724

Change-Id: I95469e4c2f4aa2bc4e6342860a9c222fb4fa7e16
3 files changed