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

Fixes #2802
Format lxc_template.go with gofmt
Add explicit test strings for new escapeFstabSpaces function
Merge pull request #2844 from tianon/fix-mount-spaces

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