Description of problem:

After running `dnf install certbot-nginx`, the Nginx plugin is not available when using the "certbot" executable placed in the user's PATH. When `dnf install certbot-nginx` is run, it appears that it installs both a Python 2 and Python 3 version of Certbot but only a Python 2 version of the plugin. The executable "certbot" is a symlink to "certbot-3", where the Nginx plugin was not installed, causing this behavior.


Version-Release number of selected component (if applicable): 0.18.1-1.fc26


How reproducible: Always


Steps to Reproduce:

It's probably easiest to use Docker the official Fedora Docker image.

1. docker pull fedora:26
2. docker run --rm -it fedora:26 sh -c 'dnf install certbot-nginx -y && certbot plugins'


Actual results:

```
Saving debug log to /var/log/letsencrypt/letsencrypt.log
* standalone
Description: Spin up a temporary webserver
Interfaces: IAuthenticator, IPlugin
Entry point: standalone = certbot.plugins.standalone:Authenticator

* webroot
Description: Place files in webroot directory
Interfaces: IAuthenticator, IPlugin
Entry point: webroot = certbot.plugins.webroot:Authenticator
```

The Nginx plugin is not listed.


Expected results:

```
...
Saving debug log to /var/log/letsencrypt/letsencrypt.log
* nginx
Description: Nginx Web Server plugin - Beta
Interfaces: IAuthenticator, IInstaller, IPlugin
Entry point: nginx = certbot_nginx.configurator:NginxConfigurator

* standalone
Description: Spin up a temporary webserver
Interfaces: IAuthenticator, IPlugin
Entry point: standalone = certbot.plugins.standalone:Authenticator

* webroot
Description: Place files in webroot directory
Interfaces: IAuthenticator, IPlugin
Entry point: webroot = certbot.plugins.webroot:Authenticator
```

The Nginx plugin is listed.


Additional info:

I'm not sure what other versions of Fedora are affected. This bug was reported to me at https://github.com/certbot/certbot/issues/5132.