neutron_verify: skip FIP ping when pre_launch.bash did not run

[rebtoor]

Problem

The FIP connectivity check in neutron_verify.yaml always ran when
prelaunch_test_instance was true, regardless of which pre-launch
scripts actually executed. When only pre_launch_ironic.bash runs
(e.g. test_with_ceph_and_ironic), no floating IP is created on the
public network, so the ping fails.

This is particularly visible in IPv6 scenarios like uni04delta-ipv6
where the public_subnet_fip_1 default (192.168.122.20) is an IPv4
address unreachable on a pure-IPv6 network, causing:

fatal: [localhost]: FAILED! => {"cmd": "ping -c4 192.168.122.20",
  "stderr": "ping: connect: Network is unreachable", "rc": 2}

Fix

• Guard the task with 'pre_launch.bash' in prelaunch_test_instance_scripts,
  matching the pattern already used in octavia_post.yaml and nova_verify.yaml
  (ironic task).
• Replace the hardcoded ping with the ping_command variable for consistency
  with pre_launch.bash.j2 and BGP scenario support.

[openshift-ci]

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[centosinfra-prod-github-app]

Build failed (check pipeline). Post recheck (without leading slash)
to rerun all jobs. Make sure the failure cause has been resolved before
you rerun jobs.

https://gateway-cloud-softwarefactory.apps.ocp.cloud.ci.centos.org/zuul/t/rdoproject.org/buildset/2ed8364e6c09436b928b257814b065a1

✔️ noop SUCCESS in 0s
❌ adoption-standalone-to-crc-ceph POST_FAILURE in 3h 08m 19s
✔️ adoption-standalone-to-crc-no-ceph SUCCESS in 3h 10m 47s

[rebtoor]

recheck