def build_host_string(user, ip, port):
host_string = "<STR_LIT>" % (user, ip, port)<EOL>return host_string<EOL>
builds a fabric ssh host string for a vagrant instance

params:
    user: user to connect to the vagrant instance
    ip: ip address of the vagrant instance
    port: port for the ssh daemon

returns:
    string: < root@vagrant_ip_address:port_number >