Fix the linking of the nginx configuration file (#458)

In this commit I'm adding some logic to ensure that the `entrypoint.sh`
file handles a pre-existing `mattermost.conf` file within the nginx
folder.

During deployment of some more custom nginx configuration it was
noticed that the linking process fails, because a file already exists
with the filesystem.

Signed-off-by: Akendo <akendo@akendo.eu>

web/entrypoint.sh

@@ -11,6 +11,8 @@ if [ -f "/cert/cert.pem" -a -f "/cert/key-no-password.pem" ]; then
 else
   echo "linking plain config"
 fi
+# Ensure that the configuration file is not present before linking. 
+test -w /etc/nginx/conf.d/mattermost.conf && rm /etc/nginx/conf.d/mattermost.conf
 # Linking Nginx configuration file
 ln -s -f /etc/nginx/sites-available/mattermost$ssl /etc/nginx/conf.d/mattermost.conf