confdb: Add new option subdomain_inherit
    
    Adds a new option subdomain_inherit that would allow administrators to pick
    and choose which option to pass to subdomains.
    
    This option is required for:
        https://fedorahosted.org/sssd/ticket/2644
    as a short-term fix.
    
    The proper solution is described in:
        https://fedorahosted.org/sssd/ticket/2599
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>
    (cherry picked from commit 1711cbfd2e36d44af1ae50e3a2beeec3a1f0b5e8)