Merge mysql55 module into mysql

This also removes the default for services.mysql.package, as this should
not generally be updated automatically if we change the mysql attribute

pkgs/servers/sql/mysql/5.1.x.nix

@@ -27,6 +27,8 @@ stdenv.mkDerivation rec {
       rm -rf $out/mysql-test $out/sql-bench $out/share/info
     '';
 
+  passthru.mysqlVersion = "5.1";
+
   meta = {
     homepage = http://www.mysql.com/;
     description = "The world's most popular open source database";

pkgs/servers/sql/mysql/5.5.x.nix

@@ -28,6 +28,8 @@ stdenv.mkDerivation rec {
     rm -rf $out/mysql-test $out/sql-bench
   '';
 
+  passthru.mysqlVersion = "5.5";
+
   meta = {
     homepage = http://www.mysql.com/;
     description = "The world's most popular open source database";