Add option to configure SSH public key in $OPENHAB_CONF

[Flowdalic]

Currently we have to modify a file under $OPENHAB_HOME to configure Karaf's SSH daemon to use public key authentication. As pointed out in the documentation, files in $OPENHAB_HOME are not meant for user configuration as they are overriden when updating to newer OpenHAB versions.

Since public key authentication is preferable over password-based authentication, it would be great if a public key could be configured for openhab's SSH console via $OPENHAB_CONF.