Add two new configuration options, RE: TCP checks

This commit adds two additional configuration items:
```
+      default_config :ssh_sleep, 120
+      default_config :no_ssh_tcp_check, false
```

If `no_ssh_tcp_check` is true, `wait_for_sshd(state[:hostname])` will be skipped after sleeping for `ssh_sleep` seconds (default sleep of 2 minutes). These changes are required until test-kitchen can read complete SSH configurations, including any proxies or other overrides.

Exposing this new option isn't specific to Rackspace. These settings have been added to control a behavior in kitchen-rackspace (calling a method in test-kitchen that performs a TCP check) that otherwise has no way for the user to control. While that method should probably be removed/fixed from test-kitchen (or made to actually test SSH functionality rather than TCP connectivity), it's actually kitchen-rackspace that is calling it (and should at least allow an override for the behavior). And it's broken for everyone who requires any kind of SSH bastion or proxy command.
Add unit tests for new functionality and update changelog
Merge pull request #32 from test-kitchen/roboticcheese/add_tcp_settings

Add two new configuration options, RE: TCP checks