Support Duo bypassing MFA requests

[pdecat]

Duo can be configured to not request MFA when connections come from specific IP addresses.

When that occurs, it directly bypasses MFA and returns the authentication cookie.

Without this PR, the following error occurs when Duo bypasses MFA:

# aws-adfs login --profile=default --region=eu-west-1 --adfs-host=sso.mydomain.com --ssl-verification --session-duration 14400
Sending request for authentication
2020-07-21 13:29:26,882 [authenticator authenticator.py:authenticate] [569451-MainProcess] [139864343611200-MainThread] - ERROR: Cannot extract saml assertion. Re-authentication needed?
Username: me@mydomain.com
Password: 
Sending request for authentication
2020-07-21 13:29:39,260 [authenticator authenticator.py:authenticate] [569451-MainProcess] [139864343611200-MainThread] - ERROR: Cannot extract saml assertion. Re-authentication needed?
This account does not have access to any roles

This has been tested by a coworker and myself from non-specific IP addresses for which MFA is required, and specific IP addresses for which MFA is not required.

[pdecat]

Internal testing went well, ready to merge IMO.

[pdecat]

Rebased on master to test Github Actions.