Log Message:
Update security/py-certbot to 0.18.0.

### Added
- The Nginx plugin now configures Nginx to use 2048-bit Diffie-Hellman
  parameters.

### Changed
- certbot-auto now installs Certbot in directories under `/opt/eff.org`.
- The Nginx plugin can now be selected in Certbot's interactive output.
- Output verbosity of renewal failures when running with `--quiet` has
  been reduced.
- The default revocation reason shown in Certbot help output now is a
  human readable string instead of a numerical code.
- Plugin selection is now included in normal terminal output.

### Fixed
- A newer version of ConfigArgParse is now installed when using
  certbot-auto causing values set to false in a Certbot INI
  configuration file to be handled intuitively.
- New naming conventions preventing certbot-auto from installing OS
  dependencies on Fedora 26 have been resolved.