Add support for X11 configuration

[bajzekm]

Is your feature request related to a problem? Please describe.
Several parameters within the templates for openssh.conf and opensshd.conf are hardcoded such that they can't be modified using this role. It would be better if all of those values were configurable.

In particular, my use case calls for enabling X11 on some systems (via ForwardX11, X11Forwarding, and X11UseLocalhost).

Describe the solution you'd like
Instead of hardcoded lines like ForwardX11 no, change the template files to use variables (something along the lines of ForwardX11 {{ ssh_x11_forwarding }}) and set those variables to default values matching the current hardcoded settings.

Describe alternatives you've considered
ssh_custom_options and sshd_custom_options are both provided in defaults/main.yml, but this is an insufficient option, since it yields configuration files with lines that contradict one another.

[rndmh3ro]

Fixed by #300