Migration now accepts scope as argument
    
    Adds a new option to command ipa migrate-ds, --scope=[base,onelevel,subtree]
    which allows the user to specify LDAP search depth for users and groups.
    'onelevel' was the hard-coded level before this patch and is still
    default. Specify 'subtree' to search nested OUs for users and groups.
    
    https://fedorahosted.org/freeipa/ticket/2547
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>