Allow alternative executables

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
```