Update config.template for Cassandra 2.1.0

the latest cassandra has removed support for various parameters,
including (but not limited to): multithreaded_compaction,
memtable_flush_queue_size, preheat_kernel_page_cache,
in_memory_compaction_limit_in_mb, and compaction_preheat_key_cache.
this patch updates config.template to be the default that's
included as a part of a 2.1.0 install.

Change-Id: I2ea9a53311b3a7a82bac016bea14ec1f52471ab7
Closes-Bug: #1376951