Fix for TACACS on Cumulus Linux

[ericpulvino]

Fixed issue where av_pairs specified in do_auth.ini were being ignored.

[helpdeskdan]

Thanks for the fix. Jathan has dropped off the earth. (I'm the original author) Would you be willing to help with more fixes?

[jathanism]

I'm back! I just suck at email. Thanks for the contribution. I'd like to go ahead and merge this, but I'd like it if the new change was commented.

Can you please add a brief comment in the code about what this change is supposed to be doing?

Thanks!

[jathanism]

Also can you please rebase this against the develop branch? I'd like to give this project a little love and bring it up to speed by using this branch to filter stable changes into master. Thanks!