Get config path by a string in it

Iterate over getKeys(false) on that ConfigurationSection and check if the value of that key is the one you want
Example (Java 7)

Code (Text):

private void findKeyByValue(String value, ConfigurationSection section) {
    for (String key : config.getKeys(false)) {
        if (config.getString(key).equals(value)) {
            return key;
        }
    }
}

Example (Java 8)

Code (Text):

private void findKeyByValue(String value, ConfigurationSection section) {
    return section.getKeys(false).stream()
        .filter(key -> config.getString(key).equals(value))
        .findAny().orElse(null);
}

If you want to look at the root of the config, call it like this

Code (Text):

FileConfiguration config = ...
String key = findKeyByValue("YourStringValue", config);

Or get the section by the path

Code (Text):

FileConfiguration config = ...
ConfigurationSection section = config.getSection("my.players.section");
String key = findKeyByValue("YourStringValue", section);

 

If this is Java 8 related then go to the spigot plugin dev page.