public String put(final String key, final String value) {
    return this._current.put(key, value);
  }
Put the given string value for the string key. This method performs any variable substitution
in the value replacing any occurance of ${name} with the value of get("name").

@param key The key to put the value to
@param value The value to do substitution on and store
@throws IllegalArgumentException If the variable given for substitution is not a valid key in
this Props.
dss-appconn/appconns/dss-eventchecker-appconn/src/main/java/com/webank/wedatasphere/dss/appconn/eventchecker/utils/Props.java