Move SSL distribution items into a regular configuration file.

Before this change, `sed` was used to find some "magic comments" in `etc/vm.args` and slurp them into a file that is writeable by the `riak` script, so that `nodetool` could use that file as arguments to its init process, and thus be able to connect to a Riak node using the SSL distribution.

This is wholly unnecessary, given that the emulator can accept multiple `-args_file` command-line arguments. Instead, we now separate the totally-optional-and-mostly-unused SSL settings into `etc/ssl_distribution.args`, which can be used independently by `nodetool` and in conjunction with `etc/vm.args` by the normal node boot process.

In order to support upgrades, the `riak` script checks for the presence of the old `sed`-slurped file. If it is present and contains active configuration items, a warning is presented and the script exits non-zero. If the file exists and only contains comments, it is deleted. Downgrades are not explicitly supported.