better roles handling in wp plugin

Summary:
Fixes issues with roles / custom roles that used an identifier that is
different from the role name. (key vs value).

This fixes the issue that kevin mitnick reported having

Test Plan:
tested in different configurations:

All roles checked:
- both admins and wp_client users were duo protected

Just Admin role checked:
- Only admins duo protected

Just wp_client role checked:
- only wp_client users duo protected

Deleted duo_roles from wp_options:
- all roles protected

Reviewers: jim, chester, jono

CC: dev

Differential Revision: http://phab.duo/D591
mark for 1.4.1 release