Ability to roll back app/advanced config to state at beginning of test [JIRA: RIAK-1607]

[tburghart]

In replication2_ssl (and others), we call rt:update_app_config repeatedly with various SSL configuration options. A non-intuitive side effect is that the changes are merged into the config file, and any setting previously added cannot be removed. I can, and will, restructure this test so that it only adds to or overwrites the last configuration, but it would be convenient, and safer, to be able to apply the configuration changes to a known constant starting configuration.

A function along the lines of rt:reset_app_config(_Node_), that restores the affected file to its condition when the test started, would alleviate having to work around accumulated changes.