Add space-escaping to path parts of lxc.mount.entry lines in generated lxc.conf, allowing for spaces in mount point names

Fixes #2802

This PR adds a new template function of "escapeFstabSpaces", since the `lxc.mount.entry` lines are, according to the documentation, formatted according to fstab characteristics, and thus spaces can be encoded with `\040`.

Tested using:
```bash
mkdir -p 'test spaces'
docker run -i -t -v $(pwd)/'test spaces':'/test spaces' tianon/debian touch '/test spaces/some file'
ls -l 'test spaces'
```

And the output of the final command should be something like:
```
-rw-r--r-- 1 root root 0 Nov 24 20:05 some file
```