Support setting of SSHKit::Host variables

Variables passed to the `server` or `role` syntax are correctly
assigned on the SSHKit::Host instance:

    server 'example1.com', roles: %w{web app}, user: 'tomc', port: 2222
    server 'example2.com', roles: %w{web app}, keys: '~/.ssh/key'

Expect this to close issue #384