Make cassandra module and do not load it by default.

mysql-test/include/have_cassandra.opt

---cassandra=on
+--plugin-load=$HA_CASSANDRA_SO --cassandra=on

sql/sql_plugin.cc

@@ -175,7 +175,8 @@ static struct
   /* we disable few other plugins by default */
   { "ndbcluster", PLUGIN_OFF },
   { "feedback", PLUGIN_OFF },
-  { "pbxt", PLUGIN_OFF }
+  { "pbxt", PLUGIN_OFF },
+  { "cassandra", PLUGIN_OFF }
 };
 
 /* support for Services */

storage/cassandra/CMakeLists.txt

@@ -22,5 +22,5 @@ STRING(REPLACE "-fno-exceptions" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
 STRING(REPLACE "-fno-implicit-templates" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
 #LINK_DIRECTORIES(/home/psergey/cassandra/thrift/lib)
 
-MYSQL_ADD_PLUGIN(cassandra ${cassandra_sources} STORAGE_ENGINE LINK_LIBRARIES thrift)
+MYSQL_ADD_PLUGIN(cassandra ${cassandra_sources} STORAGE_ENGINE MODULE_ONLY LINK_LIBRARIES thrift)
 # was: STORAGE_ENGINE MANDATORY