Improve performance of jinja2_convenience_function by not importing NAPALM when called

[glennmatthews]

Discovered this while investigating startup performance of the nautobot-server command as a part of nautobot/nautobot#4292. I found that the netutils.utils.jinja2_convenience_function() function, which is called as a part of Nautobot's startup sequence, turned out to be surprisingly expensive to call, the reason being that jinja2_convenience_function() imports (but does not itself call) netutils.lib_helpers.get_napalm_getters(), which in turn automatically imports the entire NAPALM library.

Importing napalm takes anywhere from 0.5 to 1 second all by itself, and we don't actually need that library imported until we actually call get_napalm_getters(). My solution here is to simply move the import inside the get_napalm_getters() function so that it runs when the function is called, rather then simply when the function is initially imported. This makes jinja2_convenience_function() notably faster to call.

(edit on noticing CI failure - Why are there tests that CI includes that aren't run by invoke tests? I ran invoke tests successfully locally before pushing this branch up. I'll investigate, but that's a bit annoying. 😄 )

[itdependsnetworks]

The reason is we run optional tests as these are optional dependencies. I am not sure this is the best pattern, but what we have done so far.

https://github.com/networktocode/netutils/blob/develop/.github/workflows/ci.yml#L207-L210