Dependency loop with juju/utils

[gibmat]

This library depends on some helper methods from juju/utils, but juju/utils in turn depends on juju/cmd. This forms a dependency loop.

I think the cleanest approach would be to copy over the helper code from juju/utils to break the loop. The methods used from juju/utils are:

• utils.NormalizePath()
• utils.Home()

This was noticed while reviewing dependencies needed to cleanly package LXD for Debian, where each library that LXD depends on must be able to build on its own with a clean set of dependencies.