Add 17.6.1a to cisco-8000v.gns3a

[eantowne]

Add 17.6.1a version to Cisco Catalyst 8000v to GNS3 Appliance Registry.

Before submitting a pull request, please check the following.

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When updating an existing appliance:

• [x] The new version is on top.
• [x] The filenames in the "images" section are unique, to avoid appliances / version overwriting each other.

• [x] If you forked the repo, running check.py doesn't drop any errors for the updated file.

  When creating a new appliance:

• It's tested locally, i.e.
  • [x] You dragged an instance into a project on your box, got it installed (if necessary), and did some basic network checks (ping, UI reachable, etc.).
  • [-] GNS3 VM can run it without any tweaks. - Tested on bare metal GNS3 Server installed via PPA
  • [x] The device is in the right category: router, switch, guest (hosts), firewall
  • [x] You filled in as much info as possible (checks the schemas and other appliance files for some guidance).
• [n/a] When adding a container: it builds on Docker Hub and can be pulled.
• [x] The filenames in the "images" section are unique (to avoid appliances and/or versions overwriting each other).
• [x] If you forked the repo, running check.py doesn't drop any errors for the new file.
• [n/a] Optional: a symbol has been created for the new appliance.

[grossmj]

Thanks 👍