UTIL: Convert bool mpg to an enum mpg_mode
    
    Instead of bool mpg inside struct sss_domain_info, let's introduce enum
    mpg_mode that currently maps pretty much 1:1 to the boolean. In future
    patches, a third value will be added.
    
    Also adds a getter for the mpg_mode value because we want to discourage
    getting or setting the value directly. Instead, the sss_domain_info
    structure should be opaque in the future.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3822
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>