framework: alerts should reflect plugins level
    
    Originally, an alert level was hardcoded to be "yellow" for all. With this
    patch, an alert level is created from plugin analysis levels using
    following rules:
    
    - "yellow" is default
    - "green" overrides "yellow"
    - "red" overrides everything
    
    Fixes: https://github.com/fedora-selinux/setroubleshoot/issues/12