Add support defaultNamingContext and add --basedn to migrate-ds
    
    There are two sides to this, the server and client side.
    
    On the server side we attempt to add a defaultNamingContext on already
    installed servers. This will fail on older 389-ds instances but the
    failure is not fatal. New installations on versions of 389-ds that
    support this attribute will have it already defined.
    
    On the client side we need to look for both defaultNamingContext and
    namingContexts. We still need to check that the defaultNamingContext
    is an IPA server (info=IPAV2).
    
    The migration change also takes advantage of this and adds a new
    option which allows one to provide a basedn to use instead of trying
    to detect it.
    
    https://fedorahosted.org/freeipa/ticket/1919
    https://fedorahosted.org/freeipa/ticket/2314