fix envdir to substitute {homedir}, {toxworkdir} properly.
fix envdir to substitute {homedir}, {toxworkdir} properly.

e.g.

```
[tox]
envlist = py27,py3
toxworkdir = {homedir}/.virtualenvs/my-app

[testenv:py27]
basepython = python2.7

[testenv:py3]
basepython = python3
```

maxzheng/workspace-tools