Expose TeamCityProvider.SetParameter method via the ITeamCityProvider…

… interface

Support for TeamCity's "setParameter" service message was added in
cake-build#1114 but only the tests and implementor
were updated.

This change updates the interface to match, which is what is actually exposed to
Cake scripts.

src/Cake.Common/Build/TeamCity/ITeamCityProvider.cs

@@ -52,6 +52,13 @@ public interface ITeamCityProvider
         /// <param name="buildNumber">The required build number.</param>
         void SetBuildNumber(string buildNumber);
 
+        /// <summary>
+        /// Tells TeamCity to set a named parameter with a given value
+        /// </summary>
+        /// <param name="parameterName">The name of the parameter to set.</param>
+        /// <param name="parameterValue">The value to set for the named parameter.</param>
+        void SetParameter(string parameterName, string parameterValue);
+
         /// <summary>
         /// Write the end of a message block to the TeamCity build log.
         /// </summary>