Implement Custom NTC-Templates overrides similar to TTP in #275

[jeffkala]

Environment

• Nautobot version: 2.2+
• nautobot-device-onboarding version: next

Proposed Functionality

TTP in #275 setup the datasource for /parsers/textfsm/ to allow for custom templates and/or override templates to be based in. The actual logic for the textfsm templates loading wasn't introduced yet. Needs to be introduces to look at site-packages, look in git repo and do some kind of merge.

Use Case

1. Custom textfsm templates for business logic.
2. next platform support using textfsm.

[jeffkala]

ntc-templates parse method has a try_fallback kwarg that should make this somewhat easy to implement.

parse_output(platform=None, command=None, data=None, template_dir=None, try_fallback=False) 

parse_output(platform=None, command=None, data=None, template_dir=None, try_fallback=False)