Add possiblity for multiple Napalm device credentials to be added

[tobicodesstuff]

This is a repost from Netbox #11881.

The general idea is to provide a way to use mulitple NAPALM_USERNAME and NAPALM_PASWORD credentials, possibly even on a per device basis.

The use case is to avoid having one set of credentials, as a security concern.

The suggestion from @candlerb was also quite helpful.

[xaranthos]

One possibility here, would maybe be to use i.e keepass database to keep all of the entries in, and pick from there.
Was trying to re-write some to make it work before it was split out, but been too busy to get it completed.

[candlerb]

If you're looking at external cred storage, then Hashicorp Vault is the way to go IMO. You could store under secrets/<deviceclass> either with a json blob of username + password + optional_args, or split into three.