Use “gitolite query-rc” to get gitolite variables

Instead of asking the admin to set some variables we can get them from @gitolite itself:

```
redmine@xenial:~$ for var in GL_BINDIR GL_LIBDIR; do sudo -n -u git -i gitolite query-rc ${var}; done
/home/git/gitolite/src
/home/git/gitolite/src/lib
```

Then the admin only need to define which variable to export.

Regards.