Add support for '-Dcassandra.boot_without_jna=true' for low-memory C*.

[zznate]

This patch supports running versions >= 2.1.0 withouth JNA installed in order to support running in low-memory environments like Vagrant containers, embedded systems and the like.

The underlying isssue is that JNA attempts to Malloc ~ 1024mb at minimum on startup. For those of us who like to launch multi-DC clusters using this recipe from a Vagrantfile, that is a deal killer.

See https://issues.apache.org/jira/browse/CASSANDRA-6575 and https://github.com/pcmanus/ccm/issues/97 for more info.

This patch modifies the cookcook by:

• adding '-Dcassandra.boot_without_jna=true' option to cassandra-env.sh
• removing the jna.jar from the lib directory from the recipe if enabled
• adding brief description in the readme

Fixes issue #178