Fixes #2113: NAPALM driver settings

[hSaria]

Fixes: #2113

• Allow the username and password for NAPALM to be overridden for an API call.
• Expose NAPALM's optional_args using headers prefixes with NAPALM-. Because they can contain sensitive information like secret, I've opted to use headers.
• Documented how NetBox works with NAPALM and other odd and ends.

[jeremystretch]

I'm wondering if we should prefix the header names with X-, e.g. X-NAPALM-USERNAME. Here's some discussion on the topic.

[hellerve]

I'm wondering if we should prefix the header names with X-, e.g. X-NAPALM-USERNAME. Here's some discussion on the topic.

I’m in favor of prefixing with X-. I’m usually not a fan of in-band signalling like that, but it’s become enough of a standard that I think respecting it makes sense. I understood IETF’s reasoning to deprecate it, but I do not think that those headers should ever be part of the standard, since they’re extremely specific.

[hSaria]

I've changed the prefix to X-NAPALM-