Prevent bad device token

[longbowrocks]

Description

This PR does two things:

1. It prevents gimme-aws-creds --configure from setting device_token to an invalid value.
2. It forces device registration for any user that uses clasic okta with an invalid device token set.

Related Issue

#407

Motivation and Context

I was getting double-MFAs. My config file contained device_token=.
After some pondering, I decided the default value for a feature is likely not intended to break said feature.

How Has This Been Tested?

1. gimme-aws-creds --configure && gimme-aws-creds && gimme-aws-creds without my changes. 2 MFAs on second gimme-creds.
2. gimme-aws-creds && gimme-aws-creds without my changes. 2 MFAs on second gimme-creds.
3. gimme-aws-creds --configure && sed -i'.bak' "s/device_token =.*//" ~/.okta_aws_login_config && gimme-aws-creds && gimme-aws-creds without my changes. 1 MFA on second gimme-creds.
4. gimme-aws-creds --configure && gimme-aws-creds && gimme-aws-creds with my changes. 1 MFA on second gimme-creds.
5. gimme-aws-creds && gimme-aws-creds with my changes. 1 MFA on second gimme-creds.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.