Allow different methods to find new domains for idmapping
    
    Currently the range management code is in the generic LDAP provider and
    can be used by the LDAP and AD provider. New ranges are allocated with
    the help of a hash value of the domain SID.
    
    If the IPA provider cannot find a range for a given domain it cannot
    allocate a new range on its own but has to look up the idrange objects
    on the FreeIPA server and use them accordingly. To allow the LDAP, AD
    and IPA provider to use as much common code as possible a plugin
    interface, similar to the one used to find the DNS site, to find a
    missing range would be useful. The default plugin will be used by the
    LDAP and the AD provider and the IPA provider will implement a plugin to
    read the data from the server.
    
    Fixes https://fedorahosted.org/sssd/ticket/1961