- added issue template

.gitlab/issue_templates/Bug.md

+## short description
+What is happening and what is wrong with that?
+
+## correct behaviour
+What should happen instead?
+
+## environment
+Server OS: (debian/ubuntu/centos/...)  
+Server OS version: (wheezy/trusty/centos6/...)
+ISPConfig version: (3.0.5.4/3.1.5/3.1dev/...)
+_you can use `grep 'ISPC_APP_VERSION' /usr/local/ispconfig/server/lib/config.inc.php` to get it from the command line_
+
+If it might be related to the problem  
+```
+insert the output of `nginx -v` or `apachectl -v` here
+```
+```
+insert the output of `php -v` here
+```
+
+## propsed fix
+optional, of course.  
+if you want to post code snippets, please use  
+> \`\`\`  
+> your code  
+> \`\`\`  
+
+## references
+if you know of related bugs or feature requests, please reference them by using `#<bugnumber>`, e. g. #123  
+if you have done a merge request already, please reference it by using `!<mergenumber>`, e. g. !12  
+if you know of a forum post on howtoforge.com that deals with this topic, just add the link to the forum topic here
+
+## screenshots
+optional, of course.  
+Add screenshots of the problem by clicking "Attach a file" on the bottom right.