This also fixes an issue with sourcing of /etc/rc.d/init.d/functions in
RHEL/CentOS/Fedora/etc., allowing the use of /etc/default/salt to
override environment variables.
With this commit it is possible to use virtualenv with the init scripts.
For example, I have this in my /etc/default/salt:
```
SALTMASTER=/srv/venvs/venv-1/bin/salt-master
SALTMINION=/srv/venvs/venv-1/bin/salt-minion
SALTSYNDIC=/srv/venvs/venv-1/bin/salt-syndic
SALTAPI=/srv/venvs/venv-1/bin/salt-api
PYTHON=/srv/venvs/venv-1/bin/python
```