Allow user to override single config value

[FermJacob]

Implement a new method called AddTestSettingValues to allow for 3 string (section, key, and value) with the overrideexisting bool.

public static void AddTestSettingValues(string key, string value, string section, bool overrideExisting = false)

This allows a user to not have to pass in a full Dictionary<string,string> for one value

Existing example:

public static void AddTestSettingValues(IDictionary<string, string> configurations, ConfigSection section = DEFAULTMAQSSECTION, bool overrideExisting = false)
        {
            AddTestSettingValues(configurations, section.ToString(), overrideExisting);
        }