Add TempHomeDir fixture which is already part of cookiecutter template

TempHomeDir fixture creates a temporary directory and sets it as $HOME in the
environment. Also tearDown will remove it and reverts back $HOME value to
the previous one.

1. In one of the review comments for https://review.openstack.org/#/c/55111,
   russellb suggested to add TempHomeDir fixture to test module.

2. Cookiecutter template for OpenStack based projects already has this.

Change-Id: I7bd86a9d1e1951e04a632cecb181fc1e2c30f392
Merge "Add TempHomeDir fixture which is already part of cookiecutter template"