Add function to check if reg key exists

Added functionality which allows for users to check if a reg key exists without first having to open and enumerate keys. This wouldn't work anyway because opening keys that don't exist result in failures. This new functionality just returns a boolean to indicate if the key is present on the remote host.

Commit also fixes issue where the openssh server install script was failing, as per [RM 8283](https://dev.metasploit.com/redmine/issues/8283).

There is new Meterpreter functionality required to make this work, [PR here](https://github.com/rapid7/meterpreter/pull/55).

[SeeRM #8283]