Add prompt_param method to avoid code duplication
    
    Extracted common code from ipalib/plugins/cli.py and
    ipalib/plugins/dns.py that provided way to prompt user
    for the value of specific attribute.
    
    Added prompt_param method to Command class in ipalib/frontend.py
    
    Done as part of https://fedorahosted.org/freeipa/ticket/3602