Bionic support + minor flexibility improvements

[akx]

Ubuntu 18.04 (Bionic) support

Upstream hasn't published the PPA repo for Bionic (hlandau/acmetool#307), so acquiring it fails on Bionic. However, quoting the readme:

(Note: There is no difference between the .deb files for different Ubuntu release codenames; they are interchangeable and completely equivalent.)

This means it should be safe to always use Xenial.

Nginx assumptions lessened

Not all nginx installations (e.g. those installed with https://github.com/nginxinc/ansible-role-nginx) have a sites-available/sites-enabled directory, so if the acmetool_nginx_sites_filename variable is emptied, no files are attempted to be written there.

First-class support for webroot

Webroot is now a more first-class supported method, as the path variable is exposed and can thus be used in a custom (no-magic) nginx config.

[hakamine]

@akx thank you very much for your contribution! Did a quick test and looks good. Will merge. 👍