infra: add Cisco NX-OS (n9kv) to EC2 image bootstrap

[dhalperi]

Wires the lab builder for Cisco Nexus 9000v qcow2 images so a
nexus9300v64..qcow2 dropped in infra/images/ and uploaded to
S3 builds the vrnetlab/cisco_n9kv: container on first launch
and is loaded directly thereafter.

ec2-setup.sh changes:

• Refactor image classification into a classify_image helper used by
  both the docker-tarball load path and the qcow2 build path. Add the
  Cisco mapping (nexus9300v*|nexus9500v*|nxosv*|n9kv*|nxos ->
  vrnetlab/cisco/n9kv).
• vrnetlab's cisco/n9kv Makefile globs n9kv-.qcow2; rename
  Cisco's nexus9300v64..qcow2 to that form when staging into
  the vrnetlab tree so the resulting docker tag is
  vrnetlab/cisco_n9kv:.
• Run the qcow2 build path in addition to (rather than instead of) the
  pre-built tarball path, filtered by --images and what was already
  loaded. Previously, presence of any pre-built tarball in S3 would
  short-circuit qcow2 builds for other vendors.
• Tolerate grep no-match in the post-build docker-tag lookup
  (grep ... || true) and treat upload failures as warnings rather
  than aborting the bootstrap. Without this, set -euo pipefail killed
  cloud-final after a successful build whenever the docker-tag grep
  pattern did not match.

ec2-launch.sh / README: document nxos in the --images filter values.

Smoke-tested end to end on a fresh m8i.2xlarge: built
vrnetlab/cisco_n9kv:10.3.9.M from
nexus9300v64.10.3.9.M.qcow2, deployed it under containerlab as
cisco_n9kv, and confirmed show version reports NX-OS 10.3(9) over
SSH.

---

Prompt:

I want to add Cisco VMs to our setup. For NX-OS 10, I have:

[Cisco download page listing nexus9300v64-lite/full and nexus9500v64
in .box / .ova / .qcow2 / .vagrant variants]

I don't know what to look for for IOS XE, ASA, or IOS XR

Follow-ups in the same task: picked
nexus9300v64.10.3.9.M.qcow2 (full features, KVM); confirmed
infra/ec2-setup.sh + ec2-launch.sh did not yet recognize NX-OS;
authorized wiring it up end-to-end on a fresh EC2 instance separate
from another running lab-validation session, including upload to S3,
build, smoke-test, and teardown.

---

Stack:

• infra: add Cisco NX-OS (n9kv) to EC2 image bootstrap #181 ⬅

---

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.59%. Comparing base (8048e8d) to head (8526e8c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #181   +/-   ##
=======================================
  Coverage   83.59%   83.59%           
=======================================
  Files          87       87           
  Lines        4194     4194           
=======================================
  Hits         3506     3506           
  Misses        688      688           

Flag	Coverage Δ
unittests	83.59% <ø> (ø)

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