Refactor and relocate set_subject_base_in_config
    
    Refactor set_subject_base_in_config to use api.Backend.ldap2 instead
    of a manually created LDAP connection.
    
    Also rename the function to have a more accurate name, and move it
    to 'ipaserver.install.ca' to avoid cyclic import (we will eventually
    need to use it from within that module).
    
    Part of: https://fedorahosted.org/freeipa/ticket/2614
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>